Making a request : May I…
Can I…
Example : May I borrow your pen?
Can I use the office phone?

Asking for help : Could…
Would…
Example : Could do me a the favor please?
: Would you give / lend me a hand please/

Offering a help : Can…
Example : Can I help you?

Refusing
Example : I’d love to, but i’m sorry I can’t…
: I’m afraid, I can’t…
: I’m busy my self…

Accepting
Example : That’s cool
Certainly
Of course
Terrific
What is it

Refusing a help :
Example : Well done bother
Thank anyway
That all right, I cant main it