nornen:
"Terure" is not modifying the verb. It is what you call a "predicative" or in Bertilow's Esperanto grammar a "perverba priskribo" (something that modifies the subject through the verb, but does not modify the verb itself).

Still "terure" and not "terura" is completely correct in this sentence.

You can look it up here.

Just when I thought I was understanding something you had to go and ruin it for me.

LOL.

But yeah, I don't think I really get it then. So they end in -e but they're not adverbs? It still seems to me like you would use an adjective.

Estis terura. It was terrible.

Terrible describes "it." What is "it"? The situation.

Ĝi estas terura = it is terrible. 'It' is a concrete thing or beast.
'terura'(terrible) is an adjective.
Estas terure = it is terrible. 'It' is placeholder for the subject and means nothing.
'terure'(terrible) is an adverb.

sergejm:Ĝi estas terura = it is terrible. 'It' is a concrete thing or best.
'terura'(terrible) is an adjective.
Estas terure = it is terrible. 'It' is placeholder for the subject and means nothing.
'terure'(terrible) is an adverb.

I just don't get it. I mean, if it's an adverb then it should modify a verb. So I was like, okay, "terure" is modifying esti, that's why you use an adverb.

But Nornen tells me it's not modifying esti. Instead it's doing something else to the subject, which in this case is "it," i.e. the situation, the circumstances.

So now I'm confused again.

You must understand Engish 'it' - do it is a real thing or it is only a placeholder?
If 'it' is a real thing, you replace 'it' by other word.
If 'it' is a placeholder, you can't replace it.
It rains terribly = pluvas terure.
You can't say 'rain rains ...'

sergejm:You must understand Engish 'it' - do it is a real thing or it is only a placeholder?
If 'it' is a real thing, you replace 'it' by other word.
If 'it' is a placeholder, you can't replace it.
It rains terribly = pluvas terure.
You can't say 'rain rains ...'

You see, now this is different. "It rains terribly" is a sentence that I can understand, as it is proper English.

In English though we would never say, "It was terribly," as you get if you literally translate "Estis terure." No, you would say "It was terrible."

You would only use "terribly" if you're saying something like, "I went to see a play tonight, and it was terribly long. I thought it would never end." Or you could say something like, "The team lost the game by 35 points. They played terribly."

Where did you see I said "It was terribly"?

Estas terure = it is terrible.

Even if 'it' is placeholder, you use the adjective "terrible" which relates to "it" in English.
But there is no placeholder in Esperanto, so the adverb "terure" is used.
If you dubt, do you must use"terure" or "terura", you can use "teruras".

sergejm:Where did you see I said "It was terribly"?

Estas terure = it is terrible.

Even if 'it' is placeholder, you use the adjective "terrible" which relates to "it" in English.
But there is no placeholder in Esperanto, so the adverb "terure" is used.
If you dubt, do you must use"terure" or "terura", you can use "teruras".

I didn't say you said "it was terribly." I'm am saying that is its literal English translation.

As you say, it's an adverb. In English, adverbs are -ly words. Terribly, slowly, rapidly, badly, diligently.

The dictionary itself renders the definition of terure as "awfully, frightfully, terribly."

So how is "Estis terure" not "It was terribly"? This is where I am failing to get it. I am not understanding the logic and reason behind the language. It feels random.

In the case of the general sentence: "It is terrible."

We've had this conversation before in relation to the weather, but here it is again. If the sentence is meant as a comment on the general state of things, the word "it" in that sentence doesn't refer to anything. English requires something in that position, so we stuff an "it" in there. But "it" isn't actually pointing at anything. It's grammatical filler.

In Esperanto, we don't do that. Our pronouns have to refer to things. So we drop the meaningless "it", leaving us with the impulse to just use the verb and the adjective. "Estas terura". BUT WAIT! That can't be correct, because once we drop the "it" that adjective doesn't have a noun (or pronoun) to hang off of. Adjectives are descriptive words for nouns, and therefore MUST have nouns that they describe. "Estas terura" lacks a noun. So, no nouns, no adjectives. But it does have a verb (estas), which is literally the only thing we have left to work with. What modifies (or describes) verbs? Adverbs.

"Estas" by itself refers to an ongoing state (or technically act) of being (i.e., "how things are going"). So in our sentence, what is that state of being? What word describes that state? How are things going? Well... terrible, whose root is "terur". "Terrible" + the adverbial marker (required to note that it is describing the verb, in this case "estas") = "terure".

So the correct expression is "Estas terure". It has a verb, and an adverb to describe that verb. All the parts work together properly.

The alternative is to make the whole thing a single word: "Teruras". This combines the verb and its description into something which for all practical purposes means the same thing as "Estas terure". This is also generally, I believe, the dominant method of this kind of expression.

Do NOT literally translate Esperanto adverbs by throwing an -ly on the English version. That only sorta works, and sometimes just generates nonsense. "Estas terure" does not mean "Is terribly", it means "things are going terribly" or, more shortened, "It is terrible". English adverbs and Esperanto adverbs only work similarly. They are not the same, and thinking of them as the same will only confuse you. Pay attention to the function of the words, not how they map to English. This becomes doubly important when you are dealing with words that have no English equivalents, and therefore no -ly forms to glance at for inspiration.

Adverbs describe verbs, and in some situations can stand alone. Adjectives do not work this way at all, and must have a noun to attach to. If you have a sentence with just a verb and an adjective, the sentence is improperly formed. You cannot describe a verb with an adjective.

An adverb can also often (always?) be merged with the verb :

iri per ĉevalo => iri ĉevale => ĉevaliri
pluvi torente => torentpluvi
jesi kape => kapjesi (I really saw this verb in a novel)

From this model, the transformation "esti terure => teruresti" should be correct, but in this case, "teruri" is more elegant.
(in fact, is it possible to consider that "teruri" is a kind of contracted form of "teruresti", possible because the verb is "esti" ?)

Via some cosideration of Akademio de Esperanto, you can shorten 'esti + X-a' and 'esti + X-e' to 'X-i'. But you cannot do this with the verbs, which had other meaning before this cosideration, e.g. 'Trajno rapidas' ≠ 'Trajno estas rapida'.
So 'teruras' is enough.