Hello,
I would like to know for sure what is the difference between using it's and that's in sentences like "Is that such a problem to...?" Sorry if the question sounds too stupid, but I don't have this 'feeling' of the right variant yet. And I understand that, despite similar meanings, sometimes it does matter.
I think the answer must have been described in popular references, so if anyone points me to one of them, it will probably be enough.
Thanks.