EXPLANATION
A modal verb is an auxiliary verb that can be used to change the grammatical mood of a sentence. The key way to identify a modal verb is by its defectiveness (they have neither participles nor infinitives). Modal verbs say something about the action in relation to: possibility, certainty, obligation, etc. In a way, modal verbs express the attitude of the speaker, the speaker's opinion.
The modal verbs in English are the following:
- shall/should
- will/would
- may/might
- can/could
- must
Shall
1. Shall is used in some dialects as a synonym of will
2. Shall is also used to express "intention", offering, as in a favor:
e.g. Shall I buy the newspaper today?
Should/shouldn't
Should is used to express that something is "normative", the best choice, what is morally and ethically correct, or just what is correct or better for the speaker/listener.
e.g. I should study harder
Should is also used in the affirmative as a substitute of "if" in conditional sentences:
a) If you need to submit a paper, remember to include the author's affiliation
b) Should you need to submit a paper, remember to include the author's affiliation
Will/won't
Will is used to express the future tense. The negative form is won't
In the negative form, it can be used to express "intention not to do something":
e.g. I won't come with you to the park
This meaning can be literal or can refer to the speaker's unwillingness to do something. It depends on the context.
Would/wouldn't
Would is used for the conditional tense:
e.g. If I could, I would go to the airport with you
It can also be used to express a polite request:
e.g. Would you get me a glass of water?
Would is also used to express the preterite tense, that is, a past action that was habitual.
e.g. When I was little, we would play in the park for hours
Can/could
The negative forms are: can't, cannot, can not, couldn't, could not.
Can and could can be used to express a polite request:
e.g. Can/could you get me a glass of water?
Can can be used to express "ability":
e.g. I can speak English
Can can also be used to express that something is "possible":
e.g. There can be a very strong rivalry between siblings = There is sometimes a strong rivalry between siblings
Cannot/can't in combination with "have + -ed" can be used to express the belief that something was probably not happening. With this use, it also means "disbelief":
e.g. He can't have married her, he told me she was the most horrible woman on earth!
Must/Have to
Must and have to are used to express that something is obligatory:
e.g. He must leave now if we want to be safe
Must/mustn't can be used to express a prohibition:
e.g. You mustn't smoke in public places
Don't have or needn't to is not the same as mustn't when it is used to express a prohibition. Mustn't = prohibition, don't have to= abscence of obligation. Compare:
e.g. You mustn't call her now vs. You don't have to call her now
Must/mustn't can be used to express a personal resolution:
e.g. I must give up smoking asap
May/might
May and might can be used to express a present time possibility or uncertainty:
e.g. He might be leaving right now
In this sense, could plays the same role:
e.g. John is not in the office today. He may be sick / He could be sick /He might be sick
May/might are also used to express irrelevance in spite of certain or likely truth:
e.g. He may/might be richer, but I am certainly more intelligent
Might can be used in the first person to express that future actions are being considered:
e.g. I might go to the mall later
May can also be used to express polite request (might is not so frequent with this use):
e.g. May I use your laptop?
PRACTICE
1. Let's first practice the use of needn't vs. mustn't here:
http://www.better-english.com/grammar/neednt.htm
http://www.better-english.com/grammar/neednthave.htm
2. Practice on all kind of modal verbs here:
http://www.autoenglish.org/modalverbs.htm
http://www.nonstopenglish.com/exercise.asp?exid=917
http://www.englishpage.com/modals/modalintro.html
