1. I saved the file on the computer.

Question

2. I saved the file in the computer.
3. I saved the file at the computer.
(Which preposition do I have to use?)

4. He is making a document on the computer.

5. He is making a document in the computer.

6. He is making a document at the computer.

(Which preposition is right?)

Answer 1

We usually use "on" to describe what we do with a computer.