when should I use the pronouns le and les. Could you explain me, please.
This could be a good place to start: http://www.spanishdict.com/topics/show/17
Think of it has him/ her/ them
Le tiró la pelota a Juan. (He or she threw the ball to Juan.)
You could say "le tiró la pelota" but then the sentence isn't clear. It is like saying: He or she threw the ball to him or her. You don't know the gender/ who it refers to (unless there is prior context). So you need context... "a Juan" is to Juan.
Let's try that again.
Les tirò la pelota a los maestros.
Les refers to "los maestros". They are plural, so it becomes "les".
Cómo te fue? (How did it go [for you]).
Cómo les fue? (How did it go [for them]).
Hope that helps :)
Thanks. So, but if i'm talking about things, not about people?
Me gustan pantalones verdes. Los voy a comprar.
In this case I use los or las (if femenino)?
and when its singular I use lo and la?