I've got a few beginner questions. Is it important to distinguish between amikoj and geamikoj? Also, I really don't understand the -u verb ending. I understand its use as the imperative, but I see a sentence like "Ĉu ni ekrevenu tuj?" and I don't see why that's -u instead of -us.

Maulrus:Is it important to distinguish between amikoj and geamikoj?

No. Amiko is a sex-neutral word. There is absolutely no reason to use geamikoj when amikoj does the job.

Maulrus: Also, I really don't understand the -u verb ending. I understand its use as the imperative, but I see a sentence like "Ĉu ni ekrevenu tuj?" and I don't see why that's -u instead of -us.

Ĉu ni ekrevenus tuj would mean: "Would we return immediately (if something else happened)?". It is a conditional form, while if you say "let's return immediately", that's a kind of command to yourself and the group you're talking to, which is why you use -u.

In my opinion it is not important to distinguish between amikoj and geamikoj. I generally seldom use "ge" except in cases where the root word is inherently gendered (like patro / gepatroj), or in cases where I want to emphasize that a group is mixed.

amikoj = friends [neutral, could be a mixed group, all men, or all women]
geamikoj = a mixed group of friends [definitely both men and women]

Let me digress a little beyond your question, because ge- is closely tied up with -in-, and I expect that if you're asking about ge- now, you'll be wanting to know about -in- soon.

"amiko" is a gender-neutral word, as are most Esperanto words, although speakers of some European languages in particular sometimes suppose that amiko is male. These speakers are careful to use the word "amikino" whenever talking about a woman; their own languages are highly gendered so for them, it's reflexive to differentiate between "usonano" (American person, presumably male) and "usonanino" (American woman).

However, I have found that even people who make extensive use of -in- don't make the extensive use of ge- that you might expect. Therefore they will carefully not refer to an American woman as a "usonano"; they will always say "usonanino" in this case. But if it's a large mixed group of Americans, only very rarely will you hear "geusonanoj". They normally use "usonanoj", subconsciously underscoring that "usonano" is a gender-neutral word.

Bottom line - it's a question of emphasis, and use of these gender terms is greatly influenced by the native language of the speaker. Regardless, you won't find a great many uses of ge- beyond the obviously gendered family words (gepatroj, gefratoj, etc) and the word "geamikoj" (some people, mainly Europeans, seem to consider "amiko" in the class of gendered family words, and they rigorously distinguish)

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On the verb end, "Ĉu ni ekrevenu tuj?" means "Should we get going immediately?". If you said that with an -us ending instead, it means "Would we get going immediately?"

The -us version is expressing a purely hypothetical situation. You might hear it in a case like "If John were to arrive early, would we get going immediately?" (a hypothetical situation)

The -u ending is not only for the imperative! This is really important. -u represents a 'volitive' mode (it expresses someone's wish, desire, command, request, etc). Therefore yes, it is for the imperative, but the imperative is only a small [and extreme] subsection of the meaning of the -u ending. The exact meaning is determined through context; tone of voice and punctuation will also matter in the exact English translation.

Iru! = Go!
Vi iru. You should go.
Mi petas ke vi iru. I request that you go.
Ŝi ne volas ke mi iru. She doesn't want me to go. (literally She doesn't want that I should go)

You can compare the meanings of these sentences.

Ŝi irus - She would go.
[but she is not going; hypothetically she would go if she could, or if the circumstances permitted]
Ŝi iras - She goes
[she is going or she usually goes]
Ŝi iru. - She should go.
[I want her to go, I suggest that she should go, it would be a good idea for her to go]