Should is usually followed by an infinitive without “to”: You should eat more fresh fruit. Sometimes should is used without a following infinitive: ♦ I don’t always do everything I should.
Should does not change its form, so the third person singular form does not end in “-s”: She should see a doctor about that cough.
Questions and negatives are formed without “do”: Should we come back later? ♦ You should not bring up embarrassing topics.
The negative form should not is often shortened in conversation or informal writing to shouldn’t: Those kids shouldn’t be in there.
Should is often used in tag questions: We should leave a tip, shouldn’t we?
Should has no participles and no infinitive form.
There is no past tense, but should have followed by a past participle can be used for referring to actions that did not happen or for actions that have probably happened: I should have brought an umbrella (=I did not bring one). ♦ The meeting should have finished by now (=it is likely it has ended).
When indirect speech is introduced by a verb in the past tense, should can be used as the past tense of shall: I explained that I should be too busy to see them the following day.