So, the -us verb ending is also known as the conditional tense/mood.
What that means is that the outcome is relying on whether a thing happens. If it’s unlikely, or probable, or unknown, it’ll use the -us ending. It correlates frequently with the usage of “would” in English.
Mi estus feliĉa, se mi estus riĉa.
I would be happy if I were rich.
Se ne pluvas, ĉu vi irus promeni?
If it’s not raining, would you go for a walk?
Mi dirus «jes», sed li mensogis al mi.
I would say “yes”, but he lied to me.
Now, there’s some verbs that act a bit different in the -us ending: povus, devus, and volus.
The following is taken and translated from here (esperante).
“The verb ending -us normally shows that it’s about an imaginary, not real action. But the verb forms povus, devus, and volus yet have special significance. Most often they express a real duty, ability, or will relating to the action, which still isn’t done, being done, or will be done:
Mi devus labori.
I should work. (Indeed there exists for me a duty to work, however, I don’t work.)
Mi povus foriri.
I could go away. (It’s possible for me to go away, however, I will stay.)
Mi volus helpi al vi.
I would like to help you. (I do want to help you, but that’s not possible.)
Ili iris, sed ĝuste en la direkto ĝuste kontraŭa al tiu, en kiu ili devus iri.
They went, but right in the direction exactly opposite to that which they should go.
Mi povus ĵuri al vi, ke ili estas sinceraj.
I could swear to you that they are sincere. (However, I probably won’t swear because it’s not necessary.)
Volus kato fiŝojn, sed la akvon ĝi timas.
A cat would like fishes, but it’s scared of the water.
Sometimes povus, devus, and volus still can appear with the ordinary nonreal sense of the -us ending:
En grupo respektge staris en angulo la pastraro de la ĉirkaŭaĵo, kvazaŭ devus okazi enterigo, sed tie estis gaja festo, ĝi nur ankoraŭ ne sufiĉe ekmoviĝis.
The clergy of the neighborhood stood reverently in a corner in a group, as though a funeral should happen, but there was a festive celebration, it wasn’t enough to move. [<- that last bit is hard for me to translate, sorry].
(The duty was only an appearance.)
If it’s about an action which one hasn’t done, it’s not necessary to use povus, devus, or volus. If the context clearly shows the sense, one can also use an -is ending:
Mi volis lin bati, sed li forkuris de mi.
I wanted to hit him, but he ran away from me. (I did not succeed in hitting him.)
Nun la samovaro devis kanti, sed ĝi senkulpigis sin per malvarmumo.
Now the samovar had to sing, but it excused itself with a cold. (The samovar did not sing.)
La ĉambelanoj, kiuj devis porti la trenaĵon de la vesto, eltiris siajn manojn al la planko, kvazaŭ ili levas la trenaĵon.
The chamberlains, who had to carry the train of the dress, stretched their hands to the floor as though they carried the train. (The chamberlains did not carry the train because it did not exist. That’s what’s already known.)
La hundo tuj malaperis, kaj antaŭ ol la soldato povis pensi pri tio, li vidis ĝin jam returne kun la reĝidino.
The dog immediately disappeared, and before the soldier could think about that, he saw it already returning with the princess. (He didn’t think about that.)”
The conditional mood is a bit tricky to get the hang of, so don’t feel bad if you can’t immediately understand or use it correctly. It still confuses me! It seems very simple until you try to use it, haha.
Relevant: one of my favorite Esperanto songs uses the mood: povus esti simple, sed estas ne. It could be simple, but it’s not.
