MODAL VERBS

    

- All the auxiliary verbs except be, do and have are called modals.

- Unlike other auxiliary verbs modals only exist in their helping form; they cannot stand alone as the 

  main verb in a sentence.

- It has functions such as suggesting possibility, probability, necessity, permission, obligation, 

  prohibition,or ability.

MODAL VERBS OF DEDUCTION

- must have (participle) is used to express a certainty that something happened in the past, normally by deduction. For example: "There are shards of glass on the inside, the window must have been broken from the outside!"

- may/might/could have (participle) are used to express deduction about a possibility. For example: "He may have been drinking last night because he looks terrible today!"

- can't/couldn't have (participle) are used to express past impossibility by deducation. For example: "He can't have studied much for the exam because he failed horribly."

- should/shouldn't have are used to express criticism about something in the past, or to express an opinion that something should have been done differently. For example: "I shouldn't have had that last beer, I have a terrible headache today!"