When is it proper to use "sa" vs. "sa a". Is "sa a" only used when it appears at the end of the sentence?

Sa a = demonstrative indicator (sa) +  the definite article (a)
We say sa a when it's used as a demonstrative adjective or a pronoun to identify a specific person or thing.  It may very well be translated as this one or that one.

Sa a is used for singular demonstrative adjectives
this car - machin sa a
this book - liv sa a
that person - moun sa a

In the plural form, the singular definite article "a" will turn into the plural definite article "yo"
these cars - machin sa yo
these books - liv sa yo
those people - moun sa yo

Sa a is also used for singular demonstrative subject pronouns.
Sa a pi bon - This one is better
Sa a pa mi - That one is not ripe
Sa a te tonbe nan dlo.  Li mouye. - This one fell in the water. It's wet.

In the plural form, the singular definite article "a" will turn into the plural definite article "yo"

Sa yo pi bon - These are better
Sa yo pa mi - Those are not ripe.
Sa yo te tonbe nan dlo. Yo mouye. - These fell in the water.  They are wet.


Sa a is also used for singular demonstrative object pronouns
Mwen vle sa a. - I want that one.
Kite sa a pou mwen. - Leave this one for me.
Mwen pa renmen sa a. - I don't like that one.

In the plural form, the singular definite article "a" will turn into the plural definite article "yo"

Mwen vle sa yo.  I want those ones.
Kite sa yo pou mwen. - Leave these for me.
Mwen pa renmen sa yo. - I don't like those.


Sometimes, we do write negligibly and you don't see the article "a" in our sentences.
example:
We write:
Kisa sa vle di?
instead of
Kisa sa a vle di?
What does this one mean? / What does that mean?

We also write:
Kisa sa ye? instead of Kisa sa a ye?
Sere sa pou mwen instead of Sere sa a pou mwen.
etc...
and that is ok.


Here are the ircumstances, below, when we should use Sa instead of Sa a:
When sa is contracted down from kisa, and it translates what, which, whatever, whichever,
We DO NOT / CANNOT write Sa a.

examples:
Sa ou genyen?
Kisa ou genyen? 
What's wrong?

Sa ou bezwen?
Kisa ou bezwen?
What do you need?

Sa'w panse?
Kisa'w panse?
What do you think?

Sa sa a vle di?
Kisa sa a vle di
What does that mean?

Sa ou vle, mwen vle tou.
Whatever you want, I want too.

Sa ou fè a pa bon.
The thing that you did is not good.
What you did is not good.


Also, when sa translates It is, it's, this is, that is, that (unspecific)

How's it going?
Kouman sa ap mache?

Sa fè'm mal.
It pains me.
It hurts me.

Si sa pa mache, nou nan ka.
If that doesn't work, we're in deep trouble.

Pa fè sa.
Don't do that.

It's weird. /This is strange.
Sa dwòl.

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