MODALS
Be to
obligation (formal)
Visitors are to leave the museum at 6pm.
future events
I was to go on holiday tomorrow.
I am to go on holiday tomorrow.
Need
unnecessary but not performed action
You didn't need to tell her since she already knew the result.
unnecessary and performed action = waste of time
It's raining now! I needn't have watered the garden.
Might
doubt (it's possible but unprobable)
She might come tonight.
hypothetical possibility (in the past)
I might have opened the door, but it was locked.
May
doubt ( = it's possible)
She may come tonight.
offers (polite)
May I take your coat?
permission / request permission (very polite)
May I go out with my friends? Yes, you may go out.
Ought to
Should
advice (weak obligation)
You should / should't work harder.
prediction
We should earn more money next year.
Have something done
passive meaning
I will / am going to / had / have my car repaired.
Have to
no obligation (negative)
You don't have to use all the boxes.
obligation (external)
You have to pay your taxes.
Must
belief
After such a day, you must be tired.
prohibiton (negative)
You mustn't touch the paintings in a museum
obligation (self, internal)
I must tell you the truth.
Could
Help (polite)
Could you help me?
After such a quiet day as yesterday, you couldn't be tired.
ability (in the past)
They could / couldn't speak Chinese when they were children.
They were (not) able to speak Chinese when they were children.
request permission (polite)
Could I go out with my friends?
possibility (in the past)
I could / couldn't open the door. It was locked.
I was (not) able to open the door.
Can
Help
Can you help me?
disbelief (negative)
After such a quiet day as today, you can't be tired.
ability / inability
They can / can't speak Chinese
They will / won't be able to speak Chinese after 1 lesson.
request permission / permission / refusal of permission
Can I go out with my friends? Yes, you can. / No, you can't.
I was (not) allowed to go out.
possibility / impossibility
I can / can't open the door. It's locked.
I will / won't be able to open the door.
