Modal verbs

Must

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

• I must go.

Structure of Must

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

subject + must + verb

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

Look at these examples:

subject	auxiliary must	verb
I	must	go	home.
You	must	visit	us.
We	must	stop	now.

Like all auxiliary verbs, must CANNOT be followed by to. So, we say:

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

Use of Must

In general, must expresses personal obligation. Must expresses what the speakerthinks 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.

It is sometimes possible to use must for real obligation, for example a rule or a law. But generally we use have to for this.

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)

We cannot use must to talk about the past. We use have to to talk about the past.

Have to

We often use have to to say that something is obligatory, for example:

• Children have to go to school.

Note that we can use the have to expression in all tenses, for example:
I have to, I had to, I have had to, I will have to

Structure of Have to

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

subject + auxiliary verb + have + infinitive (with to)

Look at these examples in the simple tense:

	subject	auxiliary verb	main verb have	infinitive (with to)
+	She		has	to work.
-	I	do not	have	to see	the doctor.
?	Did	you	have	to go	to school?

Use of Have to

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:

	subject	auxiliary verb	main verbhave	infinitive
past simple	I		had	to work	yesterday.
present simple	I		have	to work	today.
future simple	I	will	have	to work	tomorrow.
present continuous	She	is	having	to wait.
present perfect	We	Have	had	tochange	the time.
modal (may)	They	May	have	to do	it again.

Should

Should is an auxiliary verb, a modal auxiliary verb. We use should mainly to:

• give advice or make recommendations
• talk about obligation
• talk about probability and expectation
• express the conditional mood
• replace a subjunctive structure

Structure of Should

subject + should + main verb

The main verb is always the bare infinitive (infinitive without "to").

	subject	auxiliary verb	main verb
+	He	should	go.
-	He	should not	go.
		shouldn't
?	Should	he	go?

Notice that:

• Should is invariable. There is only one form of should.
• The main verb is always the bare infinitive.

The main verb is always the bare infinitive. We cannot say:
He should to go.

There is no short form for should. The negative should not can be shortened toshouldn't.

Use of Should

should: Giving advice, opinions

We often use should when offering advice or opinions (similar to ought to):

• You should see the new James Bond movie. It's great!
• You should try to lose weight.
• John should get a haircut.
• He shouldn't smoke. And he should stop drinking too.
• What should I wear?
• They should make that illegal.
• There should be a law against that.
• People should worry more about global warming.

We use should to indicate that we think something is probable (we expect it to happen):

• Are you ready? The train should be here soon.
• $10 is enough. It shouldn't cost more than that.
• Let's call Mary. She should have finished work by now.

We often use the conditional structure "If I were you I should..." to give advice.

• If I were you, I should complain to the manager.
• If I were you I shouldn't worry about it.
• I shouldn't say anything if I were you.

Note that we can omit "If I were you..." and just say:

• I should complain to the manager.
• I shouldn't worry about it.
• I shouldn't say anything.

If we don't understand (or agree with) something, we may use "Why should..?":

• Why should it be illegal to commit suicide? It's your life.

"Why should..?" and "How should..?" can also indicate anger or irritation:

• "Help me with this." | "Why should I?"
• "Where are my keys?" | "How should I know?"