why we used the suffix "T" withe clean although VATTEN is singular and takes EN !!!!!!?
Vatten is an ett-word. Vatten is the indefinite form, vattnet the definite. Therefore, ren becomes rent.
Yes, it's the same: vatten.
It's commonly used as a mass noun though, and the plural is rare.
It can be used to mean "a body of water" though, in which case the plural might be usable.