“I'm sure it's fine.”
You cooked some meat for dinner, and your wife told you to make sure that it is completely cooked in the middle. You say this because you think it is fully cooked.
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)