I know this is easy, but I'm having a complete brain-fart on how to do
it.

I've got a column of values on one sheet, and I want to match each
individual value in that column with the same value on another sheet,
then return the value in that same row.
Simplified example:

Column A Column B Column C Column D
a a 1 1
a b 2 1
d c 3 4
e d 4 5
c e 5 3
b f 6 2
c g 7 3
c h 8 3

So I've got values in Column A which I'm trying to match to a list of
values in Column B. When it matches, I want it to look up the
corresponding value (same row) in Column C, returning that value in
Column D (the above table is filled out as to how it should look when
done).
I think I can do this with some combination of LOOKUP, MATCH, and
INDEX, but I can't seem to get the syntax correct.

TIA!!