How do you know when to use Sa, Sak, and Sa k'ap? Sak seems redundant.

"Sa" (contraction for "kisa")

"Sa'k" (or "sa ki") are contraction for "Kisa ki" basically translates "what" when used as a interrogation pronoun.
So the three are the same.

pa egzanp, ou kapab di:
1.
Kisa ki genyen?
or
Sa ki genyen?
or
Sa'k genyen?
What's going on?

2. 
Kisa ki pase w?
or
Sa ki pase w?
or
Sa'k pase w?
What's going on with you?

3.
Eske ou konnen kisa ki te fè m fache?
or
Eske ou konnen sa ki te fè m fache?
or
Eske ou konnen sa'k te fè m fache?
Do you know what pissed me off? 

By the same token, adding "ap" indicates the progressive form:
"Sa k'ap" (or "sa ki ap") is contraction for "Kisa ki ap".
pa egzanp, ou kapab di:
4.
Kisa ki ap fè tout bri sa a?
or
Sa ki ap fè tout bri sa a?
or
Sa k'ap fè tout bri sa a?
What is making all that noise?

Other examples (with different tenses)
5.
Kisa ki ta fè w kontan?
Sa ki ta fè w kontan?
Sa'k ta fè w kontan?
What would make you happy?

6.
Nanpwen moun ki konnen kisa ki pral pase demen?
Nanpwen moun ki konnen sa ki pral pase demen?
Nanpwen moun ki konnen sa k pral pase demen?
No one knows what will happen tomorrow?



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