The Correct Way Of Saying JazakAllah Khayr
Bismillaahir-Rahmaanir-Raheem
The correct way of saying May Allaah reward you
Jazaak Allaah khayr literally means: *May Allaah reward you to a male.
Realistically, jazaak should be pronounced as jazaaka but the a is usually omitted when speaking to a male (this is the case with many spoken Arabic words), but left when written. On the other hand if jazaak were to be written in Arabic, it would be unclear who the writer is referring to, is it a male or a female? Also, Allaah should be said as Allaahu. This is due to grammatical reasons, where the u displays the doer (the one who will be rewarding), i.e. Allaah (swt).
Jazaak comes from the root word jazaa which according to the popular Arabic-English dictionary, Al-Mawrid, has two meanings that are completely opposite to eachother!
follows is the correct way of saying May Allaah reward you with good in different grammatical forms, i.e. to a man, woman, group etc.:
To a male: Jazaak Allaahu khayran
To a female: Jazaaki-llaahu khayran
To two people (whether male/female): Jazaakuma Allaahu khayran
To three men or more: Jazaakum Allaahu khayran
To three women or more: Jazaakumunna-llaahu khayran
In the case of a group of people, with only one man but many women, one must use Jazaakum Allaahu khayran (i.e. that which is used to three or more men). That is just how the Arabic grammar is.
This concludes the topic at hand inshaa Allaah. Whatever good that has been written is from Allaah. And whatever bad or mistake has occurred therein, is from me and Shaytaan.
Assalaamu alaykum wa rahmatullaahi wa barakaatuhu