can you un-contract sa kap fet! and break it down word by word please?

Sa kap fèt  → Kisa ki ap fèt

Sa - is contraction for kisa which means what.

Ki - is a relative pronoun that means that, who, or which

ap - is the determiner for the progressive tense

fèt (v.) - as a verb means to do, to happen, to occur

Kisa ki ap fèt? - literally means "what that happening?"

Kisa ki ap fèt? - is the expression for "what's happening?"
The verb to be (se) is not written in this expression. 

It is not required.

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Same goes for "sak pase?"

Sak pase? → Kisa ki pase?

literally "what that passed"

Sak pase? - is the expression for "what's going on?" "What is happening" "What is up?"

Ask me anything