What do you think he is doing?
1. I think he is playing computer games on the computer.
2. I think he is playing a computer game on the computer.
(Which answer is right? Do we have to use the singular form or the plural form?)
If you use the first sentence, you are speaking in general.
If you use the second sentence, you are speaking specifically about what "he" is doing at the moment.
Both answers can be correct depending on the context.
In the first answer, "computer games" is in the plural form, suggesting that he is playing multiple games on the computer simultaneously or interchangeably.
In the second answer, "a computer game" is in the singular form, indicating that he is playing one specific game on the computer.
To determine which answer is more appropriate, you would need more information about the situation. If it is known or implied that he typically plays multiple games at once, then the first answer with the plural form would be more accurate. If there is no specific indication of multiple games, then the second answer with the singular form would be more suitable.