-is = past
-os = future
Note: In English, sometimes part of a sentence is expressed in the present tense, even though the event actually takes place in the future. In Esperanto, both parts of the sentence are in the future tense, since they happen then.
Kion vi faros, kiam vi estos gepatroj?
What will you do when you are parents?
Ni iros al la drinkejo ĉi-vespere.
We are going to the bar tonight.
ONE WORD SENTENCES
There are a few verbs in Esperanto that can form a complete sentence on their own, without a subject. Most of these verbs are relating to weather:
Pluvas. = It is raining.
Neĝas. = It is snowing.
The preposition post means "after" and is usually followed by a noun:
post la matenmanĝo
post la oka horo
after eight o'clock
However, if you want to use post with a verb, you have to use post kiam:
Post kiam ni matenmanĝis...
After we [had] had breakfast...
Post kiam mi laboris, mi dormis.
After I [had] worked, I slept.
In the same way, antaŭ ol needs to be used before verbs.
Ni manĝis antaŭ ol li alvenis.
We ate before he arrived.