How to say "Thank You" in Arabic

let's separate the word into two parts: 'jazak' and 'Allah' 

That's how it sounds to you right? (the way you spelled it is the way you hear it) 

the first word in arabic is jazaka.  (not jazak) but because 'Allah' begins with 'a', it sounds like we just use the 'a' in allah to recite 'jazaka'

In arabic, when we refer to a person we use masculine or feminin nouns/verbs according to whether the person is male of female.

So when we say jazaka allahu khair.  the verb we used is 'jazaka' (masculine)

if we speak to a female the verb will be feminine: jazaki

"jazaki allahu khair" = "Allah reward you" (female)

When we use plural form, it's Jazakum allahu khair. = "Allah reward you all"

So there you have it.  masculine, feminine, and plural form