possibility : I can finish the work tomorrow.
permission : Can I use your phone ?
ability : Dora can sing.
offer : Can I get you something ? / I can get you some water.
request : Can you help me ?
conclusion : you can't be hungry, you've eaten a lot.
possibility : We could find a new apartment.
ability : When he was younger he could speak Italian.
permission : Could I see your passport, please ?
offer : I could help you.
request : Could you wait for me ?
obrigation / necessity : We must finish this project.
conclusion : The weather is changing. It must rain later.
prohibition : You mustn't smoke in here.
advice / recommendation : You should study hard.
You shouldn't eat fatty food.
permission : May I help you ?
possibility : I may go to the concert but I'm not sure.
possibility : It might rain tomorrow. It's uncertain.
permission (past) : The boss said we might leave earlier.
obligation / necessity : I have to go to work early.
Tom has to see his dentist.
They had to cancel their trip.
We will have to save money.
She has had to work later.
no necessity : I don't have to get up early tomorrow.
He didn't have to cook yesterday.
Mr. Smith won't have to drive to work.
Tim hasn't had to work on saturday lately.
Have to = Must = Need = Have go to ( obligation / present )
Don't / Doesn't have to = Needn't = Haven't / hasn't go to