"I am against him."
Translation:Sto contro di lui.
"Stare" usually indicates maintaining a position, and that meaning can be expressed with "essere" as well; in some tenses "stare" is also used an auxiliary verb (sto imparando, I'm learning) or to cover missing conjugations of essere (sono stato, I've been).
In a few cases stare and essere carry different meaning ("sto con lei" means "she's my girlfriend", while "sono con lei" is simply "I'm with her") and in some cases stare is the only verb you can use ("sta a te", it's up to you). But you should take these examples as idiomatic.
Hmmm... I think I understand... Idioms take so much longer to be familiar with, than the language itself... I did read that forms for "stare" are used in conjunction with the gerund form of the verb to indicate ongoing actions (I'm learning- just as you mentioned)... Thanks for the reply though...