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She could not help laughing.
She could not help but laugh.

I read somewhere that can't help means "cannot stop" and can't help but means "do only the activity, nothing else".

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I puzzled over this for some time and consulted with other native British English speakers. Our conclusion is that "I couldn't help laughing" is how you would describe an automatic reaction to something funny or amusing. "I could not help but laugh" is when laughter may not be the appropriate or expected response, with the implication that laughing is better than crying, screaming or another, more appropriate, but negative, reaction or emotion. Alternatively, "I could not help but laugh" can mean that the circumstances leading to the laughter were such that no other response was possible.

  • Nice answer! I hadn't thought of the differences you mention - but I agree with them. – TrevorD May 8 '19 at 0:10
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Could not help but is the expression. It also appears as "The dinosaurs have all but disappeared." This means they are gone. How all but means that is a bit idiomatic. The shorter version She could not help laughing is a sort of shorthand that is commonly used. I would say both are correct.

Similarly people also say I could care less when they mean I could not care less.

  • Actually, "all but" means "almost" (but specifically NOT completely); if the dinosaurs have all but disappeared, then there are still a few lurking here and there. – Hellion May 6 '19 at 17:38
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    I made a point not to try and explain that since I was not sure. I could find examples that are similarly misused. – Elliot May 6 '19 at 17:49
  • If you "were not sure", then why make the statement? The answer from @DaveH clearly describes a distinction between the two phrases. – TrevorD May 8 '19 at 0:13