How to join past continous and past simple with “but”?

Question

I'm reading about how to use "when" and "while" with past continuous and past simple tense. I quite understand about them.

But there are another examples using "but" to join past continuous and past simple tense. I don't understand about them. Here are those sentences.

1. I waved to Helen, but she wasn't looking.
2. I was watching a good movie, but I fell asleep before the end.

In this cases I didn't understand why sometimes the tense after " but' was past tense and sometimes was past continuous tense. Plus, what is the appropriate tense before "but"?

Is there rules about this? Or does it depend on verb after "but"?

For the second sentence I quite understand why we have to use past tense. For, the first example I don't really get it. Can the first sentence be " I waved to Helen, but she didn't look?"?

Answer 1

All of these are grammatical, but they mean different things. It is very similar to the case without the "but":

I looked vs. I was looking.

Do you get this difference?

EDIT

Given your response:

Yes "I waved to Helen, but she wasn't looking" is grammatical, so is "I waved to Helen, but she didn't look". They are slightly different in meaning. The former means she was looking at something else, before, during and after I waved. The latter means that my waving didn't attract her attention.