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I have several questions, please answer the ones you can.

First, is there a difference in the usage of he couldn't help but and he couldn't help doing?
For instance,
can we say - he couldn't help overhearing or he couldn't help but overhear, couldn't help but notice, he couldn't help taking/ couldn't help but take??

Second,
Which one is correct, or are they both?
1) She saw the result - the prize was hers. But should she have sacrificed everything for it?
2) Shw saw the result- the prize was hers. But was she to have sacrificed everything for it.

Third,
Is the modal usage correct?

She discovered that it could fill her with joy which might haunt her for days.
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Blue-eyed SmilerFor instance,
can we say - he couldn't help overhearing or he couldn't help but overhear, couldn't help but notice, he couldn't help taking/ couldn't help but take??
Yes, both forms are OK, and they mean the same thing. The form with "but" seems slightly more formal or literary.
Blue-eyed SmilerWhich one is correct, or are they both?
1) She saw the result - the prize was hers. But should she have sacrificed everything for it?
2) Shw saw the result- the prize was hers. But was she to have sacrificed everything for it.
These are both possible (apart from the small typos in #2). #1 suggests that it's already known that she has sacrificed everything. #2 feels as if it's not yet known whether she has sacrificed everything.
Blue-eyed SmilerThird,
Is the modal usage correct?

She discovered that it could fill her with joy which might haunt her for days.
Yes, this sentence is possible.
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Thnak you very much!!!!