Modal Verbs

Modal verbs are superverbs, they are auxiliary verbs and are used to express various meanings yet, they can not be used alone, they must always be followed by a verb in infinitive.. The modal verbs include can, must, may, might, will, would, should. They are used with other verbs to express ability, obligation, possibility, and so on. Below is a list showing the most useful modals and their most common meanings:






Modal                        Meaning                                       Example
can                             to express ability                          I can speak a little Russian.
can                             to request permission                   Can I open the window?
may                            to express possibility                   I may be home late.
may                            to request permission                   May I sit down, please?
must                           to express obligation                    I must go now.
must                           to express strong belief                She must be over 90 years old.
should                        to give advice                               You should stop smoking.
would                        to request or offer                         Would you like a cup of tea?
would                        in if-sentences                               If I were you, I would say sorry.

Modal verbs are unlike other verbs. They do not change their form (spelling) and they have no infinitive or participle (past/present). The modals must and can need substitute verbs to express obligation or ability in the different tenses. Here are some examples:

Past simple:            Sorry I'm late. I had to finish my math test.
Present perfect:      She's had to return to Korea at short notice.
Future:                   You'll have to work hard if you want to pass the exams.
Infinitive:               I don't want to have to go.

Past simple             I couldn't until I was 3 years old.

                               I wasn't able to walk until I was 3 years old.
Present perfect       I haven't been able to solve this problem. Can you help?
Future                     I'm not sure if I will be able to come to your party.
Infinitive                I would love to be able to play the piano.

Modals are auxiliary verbs. They do not need an additional auxiliary in negatives or questions. For example: Must I come?

(Do I must come?), or:He shouldn't smoke (He doesn't should smoke) WRONG.

http://esl.fis.edu/grammar/rules/modal.htm