Please explain when we should use א or not. For example, why is "zot" spelled זאת instead of זות? Should something in the pronunciation tip us off? Why does "imah" end with א instead of ה, even though it is feminine? It seems random.
This is kind of like asking why the word "cookie" starts with a C, but has a K in the middle for the same sound. Or why there's an s in the word island, or a t in castle, etc. Or why in the world there's no w in the word "one." ;)
A lot of the spelling is historical, and just has to be memorized. Kind of frustrating, but that's the way it is.