When to use a colon

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

The basic structure for should is:

Note that:

• The auxiliary verb should is invariable.

  There is only one form: should

• The main verb is usually in the base form (He should go).

Look at the basic structure again, with positive, negative and question sentences:

Note that the main verb is sometimes in the form:

• have + past participle (He should have gone.)
• be + -ing (He should be going.)

The main verb can never be the to-infinitive.

We cannot say:

There is no short form for should, but we can shorten the negative should not to