why is is suyu and not only su?
The accusative case is used for specific direct objects. This is one of those cases. If you just used "su," it would mean "I drink water."
Try reading this for some more info: https://www.duolingo.com/comment/7738396
Atī suyu içer?
I am puzzled. What is the word for 'horse'? I thought it was 'at'.
Why is that only in this case?
There is an example "Kedi su içer" and it's "su", not "suyu"
The accusative case is used for specific direct objects.
"Kedi su içer" = The cat drinks water.
"Kedi suyu içer." = The cat drinks the water.
The accusative case is never used on the subjects of sentences either :)
Why we say: At suyu ičer, and not Atı suyu içer
"At" is the subject no addition.