Question

posted by rfvv Tuesday, April 10, 2018 at 11:42pm

1. He is sitting at the computer.
2. He is working at the computer.
3. He is eating at the computer.
[Are they all grammatical? Is he doing the activities near or in front of the computer?]

Reed Wednesday, April 11, 2018 at 1:00am
Yes, all are grammatical. Yes, they all mean, whatever he is doing, he is in front of or near the computer.
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Thank you for your help.

4. He is working at the computer.
5. He is working on the computer.
[What is the difference between them?]

6. He is knocking at the door.
7. He is knocking on the door.
[What about this pair? What is the difference between them?]

Answer 1

"Working at the computer" means he is doing a task (writing a paper, making a spreadsheet, or some other job that can be completed with the help of a computer).

"Working on the computer" could mean either the same as above or that the computer is broken and he's working to fix it.

I've never heard #6, but #7 is quite commonly used.