Must

Must is a modal auxiliary verb. It has no -s in the third person singular.

·         He must go. (NOT He musts go...)

Must is followed by an infinitive without to.

§  I must get some rest.

§  You must finish the report today itself. 

§  You must not tell this secret to anyone else. 

§  Little children must not be left unattended. 

Questions and negatives are made without do.

·         Must we go now?

·         You must not worry.

Must has no infinitive (to must) or participles (musting, musted). And it has no past tense. When necessary, we use other words, for example forms of have to.

·         He will have to start coming on time. (NOT He will must ...)

·         We had to cancel the project. (NOT We musted ...)

Meaning

Must indicates that it is necessary or very important that something happens. If you say that you must do something, you mean that you have a definite intention to do something in future.

§  I must get my hair cut. 

§  I must stop smoking. 

§  We must get someone to repair the roof.

If you tell someone else that they must do something, you are emphasizing that it is a good idea for them to do that.

§  You must stop lying. 

Special uses of should

Should have + past participle

The structure should have + past participle can be used to talk about past events which did not happen.

·         I should have sent the money this morning, but I forgot.

This structure can also be used to talk about past events which may or may not have happened.

·         They should have reached home by now. It is 10 o'clock.

We can use should not have + past participle to refer to unwanted or unnecessary things that happened.

·         You shouldn't have said things like that to her.

·         It is very kind of you, but you shouldn't have bothered.

Should or Would?

In British English, both would and should can be used after first person pronouns (I and we). There is no difference in meaning.

·         I would/should like some sweets before I go to bed.

·         We would/should be happy to receive them at the airport.

Should after why

Should can be used after why to suggest surprise.

·         Why should anyone want to buy something so useless?

·         Why shouldn't she buy it if she can afford it?

·         I don't see why we should have to pay for your mistakes.

Should in subordinate clauses

Should can be used after certain adjectives expressing personal judgments and reactions. Examples are: odd, strange, sad, unfair etc.

·         It is strange that she should find old men attractive.

·         It is odd that she should want to trust him again.

·         It is unfair that she should have died so young.

This also happens after adjectives and nouns expressing the importance of an action. Examples are: necessary, important, essential, vital, eager etc.

·         It is important that the meeting should be a success.

·         It is necessary that she should be told.

Other cases

Should can used after so that, in order that, for fear that, in case and lest to show the purpose of an action.

·         He took an umbrella so that he shouldn't get wet.

·         He ran lest he should miss the train.

·         He took his umbrella in case it should rain.

Should in conditional clauses

Should is used in conditional clauses expressing possibilities, suppositions etc.

·         If he should come, ask him to wait.

·         Should it rain, there will be no picnic today.

Note that if he should come indicates less likelihood of his coming than if he comes. The sentence means something like this: There is not much chance of his coming. But if he turns up, ask him to wait.