I've come across sentences such as "it can only last for so long". The word ONLY acts as negative form, right?
Would "It can't last for so long" be an equivalent?
"Only" is not negative, but it is semantically close to a negative, in that "It can only last for so long" entails that "It won't last for very long".
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"Only" is not negative, but it is semantically close to a negative, in that "It can only last for so long" entails that "It won't last for very long".
EyeSeeYouWould "It can't last for so long" be an equivalent?
No. "only" excludes everything but the stated situation.
'for so long' indicates 'for a limited amount of time', so 'an infinite amount of time' (i.e., 'forever') is the situation that is excluded in 'only for so long'.
I'd say the equivalent is It can't last forever.
CJ