Checking the PIV for terur:

Teruro = a great fear.
Terura = a quality that causes great fear
Terure = a manner that causes great fear
Teruri (tr) To cause great fear

(http://vortaro.net/#terur)

Estas terure does seem to mean teruras. I don't think that it's a good match for the English word "terrible" though, which doesn't necessarily involve fear, to my knowledge. Honestly, for terrible I'd go with malbonega. Nevertheless, the CEED does define "terrible" with "terura" (as do some online dictionaries). That feels like an approximation to me.

(A tiny bit later) I just checked an English dictionary which said that "terrible" used to mean something that could invoke terror (which is the meaning of terur-) but that this is archaic and no longer a common usage, which is in line with what I thought. The meaning of "terrible" in modern English is just "really bad". So for a translation of "it is terrible", terur is the wrong root to use, and I would go with "malbonega" despite what the CEED says.

The CEED (or other dictionaries) may be mapping Esperanto to English using the English in use at the time Esperanto was developed (when "terrible" may have had additional meanings no longer used today) and doesn't take into account any changes to the meanings of the English words over that time. "Terrible" doesn't map to "terura" any longer, because "terur-" has to do with fear and "terrible" no longer does.

it sounds like "adverb" is not even the right term for -e words in Esperanto

In fact, the PMEG, generally accepted as the best description of Esperanto grammar around, does not use the word "adverb". It simply calls them e-vortoj.

"Adverb" is more a term from English, and is used mostly to give the general idea. It doesn't map 100% from one language to the other.

sudanglo:

And what I can't understand them is this: Are they a logical part of the Esperanto language puzzle, governed by rational rules that make sense, or are they just a wildcard that can do whatever they want, like a Joker in a card deck?

Simple rule, PM-K., if you want to qualify a noun or pronoun then use the a-form. For anything else use the e-form.

Exception: when by context or convention the missing (subkomprenita) noun/pronoun is obvious , then you can use the a-form although the noun/pronoun which is being qualified is absent. So this isn't really an exception

examples:
Ĉu vi parolas la anglan? Do you speak English? (what else could the missing noun be other than 'lingvon')
Cu vi volas la ruĝan? Do you want the red one? (said in situations where it is obvious what items are under selection)

Might be different opinions about this one

Mi trovis necesa doni kelkajn klarigojn ... (missing tion)

Hmm, okay.

I can see that this is a more complicated part of Esperanto grammar that I first expected or realized. I think it will take a while to get a handle on this, and probably a lot of exposure to examples.

Ironically, my instinct has always been to default to adjectives and shy away from -e words, perhaps because their actual function already seemed mysterious to me.

Just one example of this mystery: I really don't understand why "matene" translates into "In the morning." It would seem to me to mean something more like "morningly," even though that's not really a word in English. It seems obvious to me that to express a thought like "In the morning" you would use "En la mateno" or perhaps even "Dum la mateno" . . . but "Matene"? Total mystery.

RiotNrrd:Checking the PIV for terur:

Teruro = a great fear.
Terura = a quality that causes great fear
Terure = a manner that causes great fear
Teruri (tr) To cause great fear

(http://vortaro.net/#terur)

Estas terure does seem to mean teruras. I don't think that it's a good match for the English word "terrible" though, which doesn't necessarily involve fear, to my knowledge. Honestly, for terrible I'd go with malbonega. Nevertheless, the CEED does define "terrible" with "terura" (as do some online dictionaries). That feels like an approximation to me.

(A tiny bit later) I just checked an English dictionary which said that "terrible" used to mean something that could invoke terror (which is the meaning of terur-) but that this is archaic and no longer a common usage, which is in line with what I thought. The meaning of "terrible" in modern English is just "really bad". So for a translation of "it is terrible", terur is the wrong root to use, and I would go with "malbonega" despite what the CEED says.

The CEED (or other dictionaries) may be mapping Esperanto to English using the English in use at the time Esperanto was developed (when "terrible" may have had additional meanings no longer used today) and doesn't take into account any changes to the meanings of the English words over that time. "Terrible" doesn't map to "terura" any longer, because "terur-" has to do with fear and "terrible" no longer does.

That's good detective work there.

That's interesting to know and also useful. It makes you wonder how it even is that words can shift meaning over time like that, because I don't really think of "incredibly frightening" and "really bad" or "really low-quality" to be very near each other in terms of meaning.

RiotNrrd:it sounds like "adverb" is not even the right term for -e words in Esperanto

In fact, the PMEG, generally accepted as the best description of Esperanto grammar around, does not use the word "adverb". It simply calls them e-vortoj.

"Adverb" is more a term from English, and is used mostly to give the general idea. It doesn't map 100% from one language to the other.

Since it's been demonstrated already that they modify much more than verbs, it seems to me that English-language learning materials should perhaps not call them adverbs.

PrimeMinisterK:
Since it's been demonstrated already that they modify much more than verbs, it seems to me that English-language learning materials should perhaps not call them adverbs.

An author of learning materials must make a choice:

Use the familiar terms (adjective, adverb, conditional etc.) in order to provide some familiarity and not completely to baffle the students. But this requires stressing that some of these (like adverbs and the conditional as you have yourself discovered) are not used in the same way as in <insert your favourite reference language here>.

or

Use the PMEG terms of a-vorto, e-vorto, imaga modo etc. in order to not mix the Esperanto concepts with the familiar terms but at a risk of baffling the students (and perhaps scaring them from taking the course in the first place).

I can see the appeal of the first alternative for lernolibroj por komencantoj. Unfortunately it seems that the part requiring stressing is not stressed enough in many materials.

I really don't understanding why "matene" translates into "In the morning."

Adverbs really are the pinnacle of flexibility in Esperanto. Again: master adverbs and you've mastered a huge part of the language.

So, here's the secret of matene: in many situations you can replace prepositions with adverbs.

You can replace prepositions with the accusative case too (although I think this is mostly done with al). Because I feel the accusative case is overloaded in Esperanto, I tend to use the adverbial method more frequently. Despite the long list of things you can do with adverbs in Esperanto, for some reason to me they don't feel overloaded the way the accusative case does. Go figure. Probably related to the fact that English doesn't mark the accusative case but does tend to mark adverbs, which might be pushing me towards using adverbs in Esperanto because they feel more natural(?) That's just idle musing, though.

Anyway, as you mention, you'd probably want to say "in the morning" as "en (or dum) la mateno". And you totally can. But the alternative, which is more common, shorter and (to me, at least) seemingly just as understandable, is to replace that preposition with an -e. Matene. Means the same thing, although to some extent when you do this the meaning "fuzzes" a little because you're losing the specific meaning of the preposition you're replacing. In a lot of cases (such as with matene) the preposition is either obvious or immaterial, and due to context nothing is actually lost (I mean, you're not going to be "under the morning" or above it, or in front of it, etc., and if you are then you probably need to use the preposition explicitly to call that decidedly unusual situation out, although even then the context might be all you need).

I think there can be a sense of recurrence, where applicable, when you use the adverbial form that is missing when you use the explicit preposition or the accusative. In the morning, every morning or according to some regular frequency ("I do my exercises in the morning.": the theory is that it's regular, so adverb), as opposed to in the morning, this morning ("We'll talk about our 'relationship' in the morning.": sounds like it's gonna be a one-time thing, so preposition or accusative). This extra sense would not apply to nonrecurrent things, of course.

It would seem to me to mean something more like "morningly..."

Stop that until you've internalized what -e means. I admit: I do that translation in my head, too. But I know it's limits, use a super-expansive definition of -ly that goes way beyond where English lets me normally go, and know that what I come up with can often make no sense at all in English.

So... stop that. Do not translate unfamiliar Esperanto -e words as English words with -ly on the end. It will work sometimes but it will also lead you astray if that's the only place you go. Just learn what -e means. That's what will eventually make sense of words and usages that otherwise will be completely opaque.

RiotNrrd:I really don't understanding why "matene" translates into "In the morning."

Adverbs really are the pinnacle of flexibility in Esperanto. Again: master adverbs and you've mastered a huge part of the language.

So, here's the secret of matene: in many situations you can replace prepositions with adverbs.

You can replace prepositions with the accusative case too (although I think this is mostly done with al). Because I feel the accusative case is overloaded in Esperanto, I tend to use the adverbial method more frequently. Despite the long list of things you can do with adverbs in Esperanto, for some reason to me they don't feel overloaded the way the accusative case does. Go figure. Probably related to the fact that English doesn't mark the accusative case but does tend to mark adverbs, which might be pushing me towards using adverbs in Esperanto because they feel more natural(?) That's just idle musing, though.

Anyway, as you mention, you'd probably want to say "in the morning" as "en (or dum) la mateno". And you totally can. But the alternative, which is more common, shorter and (to me, at least) seemingly just as understandable, is to replace that preposition with an -e. Matene. Means the same thing, although to some extent when you do this the meaning "fuzzes" a little because you're losing the specific meaning of the preposition you're replacing. In a lot of cases (such as with matene) the preposition is either obvious or immaterial, and due to context nothing is actually lost (I mean, you're not going to be "under the morning" or above it, or in front of it, etc., and if you are then you probably need to use the preposition explicitly to call that decidedly unusual situation out, although even then the context might be all you need).

I think there can be a sense of recurrence, where applicable, when you use the adverbial form that is missing when you use the explicit preposition or the accusative. In the morning, every morning or according to some regular frequency ("I do my exercises in the morning.": the theory is that it's regular, so adverb), as opposed to in the morning, this morning ("We'll talk about our 'relationship' in the morning.": sounds like it's gonna be a one-time thing, so preposition or accusative). This extra sense would not apply to nonrecurrent things, of course.

It would seem to me to mean something more like "morningly..."

Stop that until you've internalized what -e means. I admit: I do that translation in my head, too. But I know it's limits, use a super-expansive definition of -ly that goes way beyond where English lets me normally go, and know that what I come up with can often make no sense at all in English.

So... stop that. Do not translate unfamiliar Esperanto -e words as English words with -ly on the end. It will work sometimes but it will also lead you astray if that's the only place you go. Just learn what -e means. That's what will eventually make sense of words and usages that otherwise will be completely opaque.

Thanks.

When I read explanations like this, it actually drives home not only how little I know about Esperanto, but also how little I know about English from a technical perspective. Because I'm like, "What's a preposition again?" I should probably brush up on my basic English grammar.

The other thing is that I guess I need to go track down some learning materials that focus on adverbs. I just looked at the adverb section here in the grammar reference and, while useful, it seems a little short.

Because I'm like, "What's a preposition again?"

Yes, they are a core concept in Esperanto, to the extent that the prepositions comprise part of the handful of words that are defined in the original sixteen rules of the language along with the numbers, the -aŭ words, the pronouns, etc. They are so important that if you have not memorized them yet, then you should go do that right now.

The reason Esperanto can get away with only two cases (nominative and accusative) is because of the prepositions. They do the heavy lifting that cases do in many other languages.