Of course

Of course means ‘without any doubt’ or ‘certainly’.

• ‘Can you help you?’ ‘Of course.’
• ‘Could you get me something to drink?’ ‘Of course.’
• ‘Do you know what I mean?’ ‘Of course.’

Of course is also used for giving permission in a polite way.

• ‘May I come in?’ ‘Of course you may.’

Of course not

Of course not means ‘no’.

Mark: Could you lend me 100 dollars?

Peter: Of course not.

Of course not is also used to refuse permission.

John: Can I borrow your car tonight?

Martin: Of course not. I need it.

That’s fine with me

This expression is used to say that you have no problems with something.

Mary: I’m giving away your old shirts to some charities.

Ricky: That’s fine with me. (= I have no problems with that.)

The expressions that’s fine by me and that’s okay with me also mean the same.

Certainly 

Certainly is also used to mean ‘yes’.

Customer: Can I try this shirt on?

Salesman: Certainly. The changing rooms are over there.

Customer: Thank you.

Sure

Sure means ‘certain’.

Jack: Excuse me. Can you help me?

Sophia: Sure. What can I do for you?

Jack: Do you know where the super market is?

Sophia: There’s one just around the corner.

Jack Thank you.

More examples are given below.

‘Will you do me a favour?’ ‘Sure. What’s it?’