He did better than his sister does.

[quirrell]

He did better than his sister does.

Is the above sentence okay? Can we compare what someone did with what someone does now?

 

[5jj]

We can but we normally have a little context.

Absent context, He did better than his sister is doing/has done is more likely in my opinion.

 

[Tarheel]

We would normally be more specific than that. For example: He did better than his sister on the math test.

 

[emsr2d2]

We would normally be more specific than that. For example: He did better than his sister on the math test.

I agree that the original lacks important context but I can't see a way for your example sentence above to compare what he did (in the past) with what his sister is doing now (present). That's what the original sentence is trying to do. Yours clearly places both maths tests in the past.

 

[Annabel Lee]

We could say:

He did better on math tests than his sister does.

 

[Tarheel]

I agree that the original lacks important context but I can't see a way for your example sentence above to compare what he did (in the past) with what his sister is doing now (present). That's what the original sentence is trying to do. Yours clearly places both maths tests in the past.

I know that. Such a comparison is, I think, normally done comparing a person to himself -- comparing himself to the way he used to be to the way he is now. Maybe he's better at it than he used to be, or maybe he's not as good at it as he used to be. (The "it" could be anything.) The person could be talking about himself or someone else.

Why you would want to compare somebody in the present to someone else in the the past I have no idea.

 

[Annabel Lee]

Why you would want to compare somebody in the present to someone else in the the past I have no idea.

When you think about it, it isn't hard to come up with occasions for doing this:

Sam's grandfather smoked a lot more cigarettes per day than Sam does.​

Einstein knew more about physics than her professor does.​

Julia Child was better at cooking than my mother is.​