When do you use a for possessive and when do you not:

"I do not find my key," sounds like something a non-native speaker of English would say. I would never, ever say that sentence.

The way we normally express this idea is, "I can't find my key."

Are 'encontrar' and 'achar' completely synonymous, or is there for instance a preference in this case to use 'encontrar'?

Acho was not accepted. Reported