I want to ask you about the usage of this phrase "can't but". The matter is that some native speakers have told me that this construction can be used only with such actions which are not fraught with volition. In other words, the following sentence is not felicitous.
- Why did you sign the contract?
- In that situation I couldn't but sign the contract.
They say that as this action (i.e. signing the contract) was intentional, hence, "can't but + verb" shouldn't be used here. "Can't but + verb" is reserved for other actions which happen unintentionally, such as: I couldn't but hear what you said: you were yelling.
My question is whether you agree with this approach. Is it really not acceptable to use "couldn't but sign" in the first example? Should it be replaced with something like "I was obliged to sign?