It seems to me that “it’s fine” or “that’s fine” is an appropriate response to formal or informal statements or questions where approval is either directly or indirectly solicited, but not otherwise. And there are contexts where saying “that’s fine” may seem to be but is not synonymous with “OK” or “alright” or “no problem,” and rather indicates that approval has been sought when it has not. (And then sometimes in these contexts boyfriends go insane and spend 10 minutes consulting style guides and the OED…)
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