Modal verbs, also called modal auxiliaries or simply modals, are a type of auxiliary verb or helping verb. English has ten modal verbs:
must ought to
Modals express the mood a verb, such as ability, possibility, necessity, or another condition. They are used with a main verb to form a sentence or a question. Modals are not conjugated, have no tense, and cannot be used without a main verb.
When used with modal verbs (except ought), main verbs always remain in the infinitive without to.
In a statement the word order is subject + modal + main verb.
subject modal main verb
They can come.
Mike should walk.
In questions, the word order changes to modal + subject + main verb.
modal subject main verb
Can they come?
Should Mike drive?
wh- word modal subject main verb
When can they come?
How could he know?
The modal can indicates possibility or ability:
Tom can help you.
Wild animals can be dangerous.
Dining out can be costly.
In questions, the modal can requests permission to do something or to ask about possibilities:
Can I help you?
Can Mike come over for dinner?
Who can answer the next question?
When can we get back the results?
Could indicates possibility or ability in the past:
I could have told you that.
It could have been a disaster.
When I was young, I could run very fast.
Could speculates about future posibilities. In the following examples could and might are synonomous.
It could / might rain tonight.
That could / might be dangerous.
In yes-no questions, could speculates about present posibilities:
Could she be the murderer?
Could this be a mistake?
It can also make a request. In these examples could and can are synonomous, but could is more polite.
Could / Can you open your window?
Could / Can you help me move this sofa?
Could indicates an option:
We could go see a movie.
I could become a doctor.
The modal could is also used to form the conditional. The conditional contains an if clause and a result clause. Could is placed in the result clause.
In these examples, could expresses hypothetical situations:
If I had time, I could play tennis with you.
We could study together, if you want to.
If it weren't raining, we could go on a picnic.
Could mentions something that didn't happen because a certain condition was not met:
If we had left sooner, we could have taken the train.
I could have passed the exam if I had studied more.
I'm glad we took umbrellas. We could have gotten soaked.
Shall and Will
The modals shall/will + main verb are used to create future tenses. These modals indicate an intention or an action that is expected to happen in the future.
When used in statements, there is no difference in meaning between these two modals; however, shall is rarely used in American English.
I will / shall close the door for you.
Tom will / shall meet us at the train station.
They will / shall leave tomorrow at 8:00.
In wh- questions, shall and will ask about options.
Who will / shall drive the car?
When will / shall I see you again?
How will / shall you get here?
What time will / shall we meet?
In yes-no questions, shall and will have different meanings.
Will asks a favor.
Will / Shall you turn off the TV?
Will / Shall you stop whining?
Will / Shall you go with me?
Will also asks for information or knowledge about somebody or something.
Will / Shall Tom ever pay you back?
Will / Shall Mars be visited by humans within twenty years?
Will / Shall you be finished soon?
Shall asks about a preference. In these examples, shall and should are synonomous. In American English, shall is rarely used; when it is, it's only in the first person singular and plural.
Should / Shall I close the door?
Should / Shall he close the door?
Should / Shall they come back later?
Should / Shall Tom bring food to the party?
Should / Shall we stay here?
May and Might
The modals may and might indicate an uncertain future action. These two modals are synonymous.
I may / might go to the park, or I may / might stay home.
This may / might be a bad idea.
It may / might rain tonight.
iMay or can gives instructions or permission.
You may / can now board the airplane.
You may / can begin the exam in ten minutes.
In yes-no questions that make a request, you can use may or can. May is more polite.
May / Can I see your driver's license?
May / Can we have some more water, please?
You can might in place of may or can, but this is extremely rare in American English.
May / Can / Might I be of some assistance?
May / Can / Might we offer you a suggestion?
The modal must indicates an obligation.
You must see this movie.
Tom must see a doctor immediately.
Must also indicates an assumption or probability.
My watch must be broken.
He must have done that before moving to Spain.
In wh- questions, must is an obligation and can be replaced with the modal should. In American English, should is much more common in these types of questions.
When should / must we be there?
Who should / must I talk to?
Must can sometimes form rhetorical questions, when you want the person to stop doing something.
Must you make so much noise? = Please be quiet.
Must he ask so many questions? = I hope he stops asking questions.
Should and Ought (to)
The modals should and ought to indicate an obligation. These two modals are synonymous.
You should / ought to call your mother.
I should / ought to go home now.
When used in questions, should asks if an obligation exists. Ought is never used in questions in American English.
Should he call her?
Should we pay now?
When should we leave?
What should I wear?
Would followed by like is a polite way of stating a preference.
I would like white wine with my fish.
We would like a room with a view.
In questions, would + subject + like is a polite request for a choice to be made.
Would you like soup or salad with your meal?
Where would you like to eat dinner?
When would Tom like this delivered?
Would can make a request sound more polite.
Come here! Would you come here?
Stop making that noise! Would you stop making that noise?
Would explains an action as a result of a supposed or real condition.
I would go with you if I didn't have to work.
If I had not had to work, I would have gone with you.
She would be surprised if you came to the party.
Tom would drive, but he doesn't have a license.
Would introduces habitual actions in the past.
When I was a student, I would go swimming every day.
When Tom lived in France, he would write me long letters.