Prepositions are the small words showing movement or position, and it pays to identify them as a family the way you did with verbs, nouns and adjectives. After one, Spanish does not say with him but with he.
The same for para (for) and de (of, from): para él, para ellas, de nosotros, de ustedes.
Two exceptions, both in the first and second person singular. After a preposition, me is mí and informal you is ti.
para mí · para ti · de mí · de ti
¿Es de ustedes? · Es para ustedes
A nice test of the distinction. It's from you guys and it's for you guys are saying something characteristic of the thing. But it's with you guys is about location, so it is a state.
This is the useful way to see it. When you use le or les, you are standing in for a to, for or from that never gets said.
Los compro (the things) · Me lo compra · Me lo compran · Me los compran
The same little words can stand in for de as well. Me lo compra is both he buys it for me and he buys it from me, totally ambiguous, and context settles it.
Hablar is to speak or talk; decir is to say or tell. It sounds /dethir/ in most of Spain.
Me hace un favor · Le hace un favor · Quiero decir · Le quiero decir / Quiero decirle · Lo quiero decir
Note the contrast: lo quiero decir is I want to say it, but le quiero decir is I want to tell him.
Con is with, and it shows up inside many Latin-English words.
Consequence is "with sequence" · convenient is "with coming" · confirm is "with signing"