You cooked some meat for dinner, and your wife told you to make sure that it is completely cooked in the middle. You think it is, so you say:
I'm sure it's fine.
They probably won't mind.
sounds a little more careful, while:
I'm sure they won't mind.
sounds more confident.
When you use this phrase in writing, you should use "that":
I'm sure that it's perfectly safe.
When you say that something "is fine", it means that there's not a problem with it. For example:
A: Is it OK if I open the window for a few minutes?
B: Yeah, that's fine.
"That's fine" or "I'm sure it's fine" is not very strongly positive. It only means that there's not a problem.
You should know that "(something) is fine" is different from "a fine (something)". A "fine ___" means something that is very high quality. You use this when talking about:
- fine wine
- fine dining
- fine Art
- fine linens (sheets on your bed)
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