QaiserMirza
Chief Minister (5k+ posts)

JazakAllah or JazakAllah Khair?
Understanding Grammar Behind The word JazakAllah
Jazaak comes from the root word jazaa' (جزاء) which according to the popular Arabic-English dictionary, Al-Mawrid, has two meanings that are completely opposite to each other!
Thus jazaa' (جزاء) can either mean reward OR punishment.
So :"JazakAllah" can may either mean "may Allah reward you" or "may Allah punish you", while
"JazakAllah Khair" means ... "may Allah reward you with the Best / Good",
So the correct way is to say Jazakallah khair & not just JazakAllah. Though someone might say that the intention by saying JazakAllah is the same like JazakAllahu khayran, if it's so, then why not use the complete wordings as used by Rasool Allah pbuh and the Sahabas (radiAllahu Anhum), which is "JazakAllahu khayran" !
Jazak Allahu Khair depending on situation:
Masculine: JazakAllahu Khair/Khairan
Feminine: Jazakillahu Khair/Khairan
Plural: Jazakumullahu Khair/Khairan
Instead of Jazakallahu khair you may also use:
Feminine: Barakallahu Feeki
Masculine: Barakallahu Feek
Plural: Barakallahu Feekum
Which means "May Allah bless you",