Learn Hardcore Esperanto: Kiu amiko venos al la festo morgaŭ? - Which friend will come to the party tomorrow?

In this sentence, kiu means which and it is used together with a noun: kiu amiko = which friend.
When kiu stands before a noun, it usually means which (or sometimes what … as in what friend).
If you use kiu alone about a person, it usually means who or which one, for example: Kiu venos? = Who / which one will come?
Kio, on the other hand, means what (thing) and is not used directly before a noun: Kio venos? = What will come? (some thing, not a person).

Why doesn’t amiko have la in front of it? Why not Kiu la amiko or Kiu la amiko venos…?

With kiu plus a singular noun, you normally do not add la: kiu amiko, kiu libro, kiu urbo.
Kiu already singles out one specific member of a known group (for example, from all your friends), so adding la would be redundant or sound wrong.
So Kiu amiko venos…? corresponds to Which friend will come…?, not Which the friend…?.
If you want which of the friends, you still say kiu amiko, because the idea that the group is known is understood from the context, not from la.

Why doesn’t amiko end in -n? Isn’t friend the object of the verb?

No. In this sentence amiko is the subject: the friend is doing the action of coming.
The structure is: Kiu amiko (subject) venos (verb) al la festo (destination) morgaŭ (time)?
Only direct objects take -n in Esperanto, for example: Mi vidas amikon = I see a friend (here amikon is the object of vidas).
Because amiko is not a direct object here, it correctly appears without -n.

Why is it al la festo and not just al festo?

La festo means the party, a specific party that both speakers know about.
Without la, festo would mean a party (any party), which doesn’t fit well in this context when you have a particular event in mind.
So al la festo = to the party, referring to a definite, known party (for example, the party we have been talking about).

Why is it al la festo and not en la festo?

Al expresses movement to / towards something: iri al la festo = to go to the party.
En expresses being in / inside something: esti en la festo = to be at / in the party.
Since veni here describes the movement of coming to the party, al la festo is the natural choice.
If you said veni en la festo, it would sound like to come inside the party, focusing on entering the interior, which is not what is normally meant.

Could we say venos al la feston with -n on festo to show direction?

Normally, no. When you already use a preposition like al to mark direction, you don’t add -n to the noun.
So al la festo is correct; al la feston is generally considered wrong or at least very non‑standard.
The -n for direction is used when there is no preposition of movement:

Mi iras hejmen. = I am going (to) home.
But with al, the direction is already clear, so you just say al la festo.

What tense is venos, and why isn’t it venas?

Ven-os is the future tense: will come.
Esperanto verb endings: -as = present, -is = past, -os = future.
Because the sentence includes morgaŭ (tomorrow), the action is clearly in the future, so venos fits best.
Using venas with morgaŭ is sometimes possible in very specific contexts (like timetables or very fixed plans), but the neutral, standard form here is venos.

Can morgaŭ be placed somewhere else in the sentence, or must it stay at the end?

Word order in Esperanto is quite flexible, especially for adverbs like morgaŭ.
You could also say:

Morgaŭ kiu amiko venos al la festo?
Kiu amiko morgaŭ venos al la festo?
Kiu amiko venos morgaŭ al la festo?
All of these are grammatically correct; the changes mostly affect style and emphasis, not basic meaning.

How would I say Which friends will come to the party tomorrow? in Esperanto?

You make both the question word and the noun plural:
Kiuj amikoj venos al la festo morgaŭ?
Here kiuj is the plural of kiu, and amikoj is the plural of amiko.
The verb venos does not change for plural subjects in Esperanto; it stays the same.

Why don’t we need ĉu at the beginning of this question?

Ĉu is used for yes–no questions, where the answer is basically yes or no:

Ĉu vi venos al la festo? = Will you come to the party?
Here, the question starts with kiu, which is a question word (an interrogative pronoun) asking for specific information.
In Esperanto, when a question starts with a ki‑word (like kiu, kio, kiam, kie, kial), you do not put ĉu in front.

How could I answer this question correctly in Esperanto?

Some natural answers would be:

Mia amiko Marko venos al la festo morgaŭ. = My friend Marko will come to the party tomorrow.
Nur unu amiko venos al la festo morgaŭ. = Only one friend will come to the party tomorrow.
Neniu amiko venos al la festo morgaŭ. = No friend will come to the party tomorrow.
If the time and place are already clear from context, you can often drop them and just say something like Marko venos.

Is the word order fixed, or can I move parts like al la festo or morgaŭ around?

The basic, neutral order is: Kiu amiko venos al la festo morgaŭ?
But Esperanto allows quite free word order, as long as the endings and prepositions make roles clear. For example:

Kiu amiko morgaŭ venos al la festo?
Kiu amiko venos morgaŭ al la festo?
Morgaŭ al la festo kiu amiko venos? (more dramatic/emphatic)
You should, however, keep kiu immediately before its noun (amiko), because kiu amiko functions together as a phrase meaning which friend.