Auxiliary Verbs "Can/Could" and "May/Might/Must"

Can

Used to express ability (to be able to do something):

• I can make jewelry.
• He can’t speak French.
• Can you open this jar?

Used to ask for permission:

• Can I use your bathroom?
• Can I leave now?
• Can I raise the volume?

Used to make requests or suggestions:

• Can I have more napkins?
• Can I have the bill?
• You can take this spot if you like.
• You can do whatever you want.

Could (past form of can)

Describes an ability that someone had in the past:

• I could swim when I was young.
• You could see the boat sinking.
• They could tell he was nervous.

Often used in auxiliary functions to express permission politely:

• Could I take this jacket with me?
• You could borrow my umbrella.
• Could you please let me pass you?
• Could I get you more water?

Used to express possibility:

• All of them could ride in the van.
• You could always stay at our house.
• Could it be true?
• This plan could really work out.

May

Used to ask for formal permission:

• May I come in?
• May I say something now?
• May I ask one question?

Used to suggest something that is possible:

• She may agree with this plan.
• They may not be happy about what happened.
• It may shower tonight.

Might (past form of may)

Used to suggest a smaller possibility than may does (actually, might is more common than may in American English):

• He might have finished it.
• I might go see a doctor.
• I might not come this time.
• It might be right.
• You might have lost it.
• The store might have been closed today.

Must

Used to express something formally required or necessary:

• I must complete the project by this week.
• The government must provide health care for everybody.
• Everyone must save the natural resources of the earth.
• The building must have a fire alarm.
• You must answer my question right now.

Used to show that something is very likely:

• He must be a genius.
• You must be joking!
• There must be an accident.
• She must be very tired.