A tree.
A
one car that? Reduction inversion
Is that (one) a car?
A in most cases is used as a susbtitute for the third person singular- it is, she/he is.
Eg. A tree- Reduction from it is a tree.
A my
girl, that. Reduction inversion from she is my
girl (that one)
A Shensea. (Introduction)
My name is Shensea. I'm a woman, Shensea.
The I and a woman are removed and one is left with a + the name.