You are talking about verbal moods--and Esperanto has three:
The conditional mood is used for such expressions as se mi povus, mi irus (if I could, I would go) and se mi estus vi, mi irus (if I were you, I'd go).
Thanks, wildan1.
"Se mi povus, mi irus." is different from "If I could have, I would have gone," isn't it?
I am wondering how to refer to the past, like, the first sentence "If Oswald had not shot ...."