Modal verbs

Can - May

Both 'can' and 'may' are used in question form to ask permission.

Examples of Asking Permission with 'May' and 'Can'

Can I come with you?
May I come with you?

In the past, especially when discussing modal verbs' grammar, 'may' was considered correct and 'can' incorrect when asking for permission by many teachers. However, in modern English it is common to use both forms and considered correct by all but the strictest of grammarians.

Can - To Be Allowed To

One of the uses of 'can' is to express permission. In the simplest sense, we use 'can' as a polite form to request something. However, at other times 'can' expresses permission to do something specific. In this case, 'to be allowed to do something' can also be used.

'To be allowed to' is more formal and is commonly used for rules and regulations.

Examples of Simple Questions:

Can I come with you?
Can I make a telephone call?

Examples of Asking Permission

Can I go to the party? => Am I allowed to go to the party?
Can he take the course with me? => Is he allowed to take the course with me?

Can - To Be Able To

'Can' is also used to express ability. Another form that can be used to express ability is 'to be able to'. Usually, either of these two forms can be used.

Examples:

I can play the piano. => I'm able to play the piano.
She can speak Spanish. => She's able to speak Spanish.

There is no future or perfect form of 'can'. Use 'to be able to' in both future and perfect tenses.

Examples:

Jack's been able to golf for three years.
I'll be able to speak Spanish when I finish the course.

Special Case of the Past Positive Form

When speaking about a specific (non-general) event in the past only 'to be able to' is used in the positive form. However, both 'can' and 'to be able to' are used in the past negative.

Examples:

I was able to get tickets for the concert. NOT I could get tickets for the concert.
I couldn't come last night. OR I wasn't able to come last night.

Must

'Must' is used for strong personal obligation. When something is very important to us at a particular moment we use 'must'.

Examples:

Oh, I really must go.
My tooth is killing me. I must see a dentist.

Have to

Use 'have to' for daily routines and responsibilities.

Examples:

He has to get up early every day.
Do they have to travel often?

Mustn't vs. Don't Have To

Remember that 'mustn't' expresses prohibition. 'Don't have to' expresses something that is not required. However, if the person may choose to do so if he or she pleases.

Examples:

Children mustn't play with medicine.
I don't have to go to work on Fridays.

Should

'Should' is used to ask for or give advice.

Examples:

Should I see a doctor?
He should leave soon if he wants to catch the train.

Should, Ought to, Had Better

Both 'ought to' and 'had better' express the same idea as 'should'. They can usually be used in place of 'should'.

Examples:

You should see a dentist. => You'd better see a dentist.
They should join a team. => They ought to join a team.

NOTE: 'had better' is a more urgent form.

Modal + Various Verb Forms

Modal verbs are generally followed by the base form of the verb.

Examples:

She should come with us to the party.
They must finish their homework before dinner.
I might play tennis after work.

Modal verbs grammar can become especially confusing when taking a look at the verbs which follow the modal verb itself. Usually, modal verbs' grammar dictates that modal verbs are followed by the base form of the verb to the present or future moment. However, Modal verbs can also be used with other forms of verbs. The most common of these modal verbs' grammar forms is the use of the modal plus a perfect form to refer to a past time when using a modal verb of probability.

Examples:

She must have bought that house.
Jane could have thought he was late.
Tim can't have believed her story.

Other forms used include the modal plus the progressive form to refer to what may / should / could be happening at the present moment of time.

Examples:

He may be studying for his math exam.
He must be thinking about the future.
Tom can be driving that truck, he's sick today.