For closing something:
* Cau: This is the most common word for "shut" and means to close something, like a door or a window.
* Cau ar gau: This means "closed shut" or "completely closed".
For closing something specific:
* Cau'r drws: This means "shut the door".
* Cau'r ffenest: This means "shut the window".
For switching something off:
* Diffodd: This means "switch off", like turning off a light or a computer.
It's important to consider the context to choose the most appropriate word.