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 Prophet(salallahu alaihi wa sallam) and the Sahabas (radiAllahu Anhum), which is "JazakAllahu khayran" !
Jazak Allahu Khair depending on situation:
Masculine:Jazak Allahu Khair/Khairan
Feminine: Jazaki Allahu Khair/Khairan
Plural: Jazakum Allahu Khair/Khairan
You may also use "JazakAllahu khairan katheeran" which means May Allah reward you with good in abundance" as "Katheeran" means "abundance/abundantly".
Instead of Jazakallahu khair you may also use:
Feminine: Baraakallahu Feeki
Masculine: Baraakallahu Feek
Plural: Barakaallahu Feekum
Which means "May Allah bless you", which is an appropriate way of thanking someone and replying to someone who's said JazakAllahu Khair to you.
It can be used both as a thanking and a reply.
As a reply you may also say:
(Below are the replies to someone who has thanked you with JazakAllahu Khair or Barakallahu Feek)
Feminine- Wa Iyyaki
Masculine- Wa Iyyak
Plural- Wa Iyyakum
They all mean, "same to you".
*They are the most appropriate replies.
Another reply could be
Feminine- Wa Anti Aydan
Masculine- Wa Anta Aydan
Plural- Wa Antum Aydan
They all mean, "you too". (Not very preferable, but it's okay to use this as a reply too).
And another reply could be:
Feminine: Jazana wa Iyaaki
Masculine: Jazana Wa Iyyak
Plural: Jazana Wa Iyyakum
Translation to it is: may Allah reward you and me too (with the same*).
*"Same" here means Khair which is good from Jazakallahu Khair or Barakah which means blessing from Barakallahu Feek.
If someone has said "Thank-You" or "Shukran" and not used "Jazakallahu Khair" or "Baraakallahu Feek" to thank you, then reply them with "Baraakallahu feek" or "Jazakallahu Khair", as replies like "wa iyyak" or "wa anta aydan" means "you too" and you can't use that as a reply as it's inappropriate.
Share this message and gain ajr and make many use these beautiful phrases.
Jazakallahu Khair !!!