how do u use usar in this sentence..(Que numero usar usted?
Qué número usar usted?
Que numero usa usted because you are speaking directly to another person. Conjugations go like this:
yo nosotros tu vosotros (spain) el/ella ellos
this would be (in order from top)
uso usamos usas usais(?) usa usan
But since saying usted is formal, you conjugate the tu version like an el/ella version, hence it is usa usted instead of usas usted.
Context is important and by the way this is a double post. You already have this questioned listed.
usa= meaning wear
What size do you wear?