The structure of can is:

• Subject + Auxiliary Verb (can) + Main Verb (infinitive without "to")

Notice that:

• Can is invariable. There is only one form of can.
• The Main Verb is always the bare infinitive (the infinitive without "to").

Possibility/Ability

We use can to talk about what is possible, what we are able or free to do:

• She can drive a car.
• John can speak Spanish.
• I cannot hear you. (I can't hear you.)
• Can you hear me?

Normally, we use can for the present. But it is possible to use can when we make present decisions about future ability.

1. Can you help me with my homework? (present)
2. Sorry. I'm busy today. But I can help you tomorrow. (future)

Requests and Orders

We often use can in a question to ask somebody to do something. This is not a real question - we do not really want to know if the person is able to do something, we want them to do it! The use of can in this way is informal (mainly between friends and family):

• Can you make a cup of coffee, please.
• Can you put the TV on.
• Can you come here a minute.
• Can you be quiet!

Permission

We sometimes use can to ask or give permission for something:

1. Can I smoke in this room?
2. You can't smoke here, but you can smoke in the garden.

(Note that we also use could/may/might for permission. The use of can for permission is informal.)

The structure of could is:

• Subject + Auxiliary Verb (could) + Main Verb (infinitive without "to")

Notice that:

• Could is invariable. There is only one form of could.
• The Main Verb is always the bare infinitive (the infinitive without "to").

Past Possibility/Ability

We use could to talk about what was possible in the past, what we were able or free to do:

• I could swim when I was 5 years old.
• My grandmother could speak seven languages.
• When we arrived home, we could not open the door. (...couldn't open the door.)
• Could you understand what he was saying?

We use could (positive) and couldn't (negative) for general ability in the past. But when we talk about one special occasion in the past, we use be able (positive) and couldn't (negative). Look at these examples:

Requests

We often use could in a question to ask somebody to do something. The use of could in this way is fairly polite (formal):

• Could you tell me where the bank is, please?
• Could you send me a catalogue, please?

The structure of be able to is:

• Subject + Main Verb (be) + Adjective (Able) + Infinitive (to do)

Notice that be able to is possible in all tenses:

• I was able to drive...
• I will be able to drive...
• I have been able to drive...

Notice too that be able to has an infinitive form:

• to be able to

Possibility/Ability

We sometimes use be able to instead of can or could for ability. Be able to is possible in all tenses. But can is possible only in the "present simple". And could is possible only in the "past simple". In addition, can and could have no infinitive form. So we use be able to when we want to use other tenses or the infinitive. Look at these examples:

• I have been able to swim since I was five. (present perfect)
• You will be able to speak perfect English very soon. (future simple)
• I would like to be able to fly an airplane. (infinitive)

Have To (for objective obligation)

"Have To" is often grouped with modal auxiliary verbs, but in fact it is not a modal verb. It is not even an auxiliary verb. In this structure, "have" is the main verb. The structure is:

• Subject + auxiliary verb + main verb "Have" + infinitive (with "to")

Look at these examples in the Simple Present tense:

In general, "Have To" expresses impersonal obligation. The subject of "Have To" is obliged or forced to act by a separate, external power (for example, the Law or school rules). "Have To" is objective. Look at these examples:

• In France, you have to drive on the right.
• In England, most schoolchildren have to wear a uniform.
• John has to wear a tie at work.

In each of the above cases, the obligation is not the subject's opinion or idea. The obligation is imposed from outside.

We can use "Have To" in all tenses, and also with modal auxiliaries. We conjugate it just like any other main verb. Here are some examples:

We often use "Must" to say that something is essential or necessary, for example:

• I must go!

"Must" is an auxiliary verb. It is followed by a main verb. The structure is:

• Subject + Must + Main Verb

The Main Verb is the base verb (infinitive without "to").

Look at these examples:

NB: like all auxiliary verbs, "must" cannot be followed by an infinitive. So, we say:

• I must go now. (not I must to go now.)

In general, "Must" expresses personal obligation. "Must" expresses what the speaker thinks is necessary. "Must" is subjective. Look at these examples:

• I must stop smoking.
• You must visit us soon.
• He must work harder.

In each of the above cases, the "obligation" is the opinion or idea of the person speaking. In fact, it is not a real obligation. It is not imposed from outside.

We can use "Must" to talk about the present or the future. Look at these examples:

• I must go now. (present)
• I must call my mother tomorrow. (future)

There is no past tense for "Must". We use "have to" to talk about the past.

"Must" is an auxiliary verb. It is followed by a main verb. The structure for "Must Not" is:

• Subject + "Must Not" + Main Verb

The Main Verb is the base verb (infinitive without "to").

"Must Not" is often contracted to "mustn't".

Look at these examples:

NB: like all auxiliary verbs, "must" cannot be followed by an infinitive. So, we say:

• You mustn't arrive late. (not You mustn't to arrive late.)

We use must not to say that something is not permitted or allowed, for example:

• Passengers must not talk to the driver.

"Must" is an auxiliary verb. It is followed by a main verb. The structure for "Must Not" is:

• Subject + "Must Not" + Main Verb

The Main Verb is the base verb (infinitive without "to").

"Must Not" is often contracted to "mustn't".

Look at these examples:

NB: like all auxiliary verbs, "must" cannot be followed by an infinitive. So, we say:

• You mustn't arrive late. (not You mustn't to arrive late.)

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