Use the verb ending -us to talk about non-real, imagined situations, or to make polite requests.
Non-real situations are the topic of "if...then" sentences, like this one:
Se mi estus sana, mi laborus. = If I were healthy, I would work.
Notice that Esperanto, unlike English, uses the -us form in both parts of the sentence - since both parts are non-real.
We also talk about non-real situations when we express our wish for something to be different from what/how it actually is:
Se li nur estus iom pli bela! = If only he were a bit more handsome!
The -us ending does not carry any temporal information. Thus, it is possible to use the -us ending for events in the past as well. When doing so, context usually indicates that we are talking about the past:
Se Zamenhof scius la ĉinan, Esperanto estus malsama. = If Zamenhof had known Chinese, Esperanto would be different.
Se vi dirus tion al mi jam hieraŭ, mi ne farus la eraron. = If you had told me this already yesterday, I would not have made the mistake.
The -us form can also be used to express polite requests.
Ĉu vi povus doni al mi la buteron? = Could you pass me the butter?
Mi ŝatus iom pli da sukero. = I would like a little more sugar.
REVIEW OF -N FOR DIRECTION
Remember to use the -n ending when talking about a movement towards a certain place. For example
Metu la dosierojn en dosierujon! = Put the files into a folder!
RETPOŜTO VS. RETMESAĜO
Retpoŝto means email in general; the service that allows you to send and receive electronic messages. A message sent by retpoŝto (email) is most often called retmesaĝo (email).