When should I use the -u verb ending?

Explanation

The verb ending -u does not show the time of the action. It is used to express a will (whether a command, a request, or a desire).
Rakontu pri via vojaĝo! - Tell us about your trip!
Diru vian nomon! - Tell us your name!

In the above examples, the verbs are used without a subject, in those cases, the second-person pronoun vi is understood.
Venu rapide! = Venu vi rapide! (Come quickly!)
Manĝu! = Manĝu vi! (Eat!)
Estu kun mi! = Vi estu kun mi! (Be with me!)
Kara, donu al mi tion. = Kara, vi donu al mi tion. (Dear, give me that.)

You can leave off the pronoun vi only in the main clause. In subclauses, you have to use it.
Legu la libron. - Read the book.
Mi volas, ke vi legu la libron. - I want you to read the book.

For the other persons (first and third), both in main clauses and in subclauses, you have to use the verb with the pronoun, otherwise people will think that you mean the second person.
Ni komencu! - Let's start!
Mi estu! - I should be!
Ĉu ni jam iru tien? - Should we go there already?
Ili mem faru tion! - They should do it themselves!
Kiel mi komprenu tion? - How should I interpret that?
Ŝi aĉetu, kion ŝi bezonas. - Let her buy what she needs.

The ending -u is used also in subclauses after the subjunctions ke and por ke, if the main clause in some way shows will, desire, aim, etc. Note that this rule applies even when the verb of the main clause has some other time.
Ili volas, ke vi laboru. - They want you to work.
Mi multe legos, por ke poste mi povu bone prelegi. - I read a lot, so that afterward I can lecture well. (So that I should be able to lecture well.)
Skribu la leteron tuj, por ke ni sendu ĝin ĝustatempe. - Write the letter immediately, so that we can send it in time. (So that we should be able to send it in time.)
Ŝiaj gepatroj petis, ke li morgaŭ vizitu ilin. - Her parents asked that he visit tomorrow. (That he should visit tomorrow.)

In order to better understand the use of the ending -u in subclauses, compare the following sentences:
Ŝi diris, ke ŝi venos morgaŭ. - She said that she will come tomorrow.
Mi petis, ke ŝi venu morgaŭ. - I asked her to come tomorrow.
Mi ĝojas, ke vi bone fartas. - I'm glad you're doing well.
Mi deziras, ke vi fartu bone. - I want you to be well. (I want that you should be well.)
Mi kuris tiel rapide, ke li ne povis kapti min. - I ran so fast that he could not catch me.
Mi volis kuri tiel rapide, ke li ne povu kapti min. - I wanted to run so fast that he could not catch me. (So that he should not be able to catch me.)

It's especially worth understanding the verb bonvolu. It is used to express a will or desire in a gentler way. It's like saying "please" in English. After the verb bonvolu, always use the infinitive form of the verb.
Bonvolu sidi ĉi tie! - Please sit here!
Bonvolu veni morgaŭ - Please come tomorrow!
Bonvolu esti ĝentila! - Please be nice!

Exercises

Level 1 Level 2