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Grammar and Vocabulary
How to Use Verbs
A guide to modals and how to use them
Modals are a special kind of helping verb, used to show possibility, probability, and necessity. They:
Like other helping verbs, modals are always followed by a second verb. But the second verb follows a different conjugation pattern if a modal is present. The second verb can never add “-s”, “-es”. “-ed” or “-ing.” It also cannot be in the infinitive form (“to …”) or in the gerund form (“…-ing”).
Some modal verbs appear to have past tense forms (could, should, might), but these are not usually used with a past meaning. One exception is “”could,”” which, when talking about ability, is used as a past form of can. (“”I could run a long way when I was younger.””)
Most modal verbs can be used in some of their meanings with a perfect infinitive to talk about the past:
When using modals, the sentence structure will be:
SUBJECT → MODAL → VERB 2 → THE REST OF THE SENTENCE
(noun, pronoun, or noun phrase) → (should, would, could, may, might, etc.) → (second verb) → (adj, adv, noun, prepositional phrase, etc.)
Shall can be used with first-person singular (I) and first-person plural (we). However, it is less common than will, especially in American English.
Shall can be used with I and we, but is less common than will, especially in American English.
In British English, first-person questions expressing willingness or wish use shall (Shall I/we? = Do you wish me/us to…?) First-person statements use will (I/we will).
Note that shall is not usually used in this way in American English.
Could is used to talk about ability, NOT about particular events which actually happened in the past. Verbs like manage to are used instead. “She finally managed to pass the exam.”
Polite requests are often made by appearing to ask about ability with can and could.
Can is commonly used to ask for or give permission. May is more formal.
Could and might are used to ask for (not to give) permission. They are more tentative than can.
Would is commonly used in the main clause of conditional sentences to show that a situation is unreal or tentative.
Because it can express tentativeness, would is also used in polite invitations, offers, and requests.
Could suggests that something is less likely than may or might.
When it expresses possibility, can is most often used in question forms: What can have happened? However it is also used to express general possibility in sentences where its meaning is similar to “sometimes”: His behavior can make us laugh. (= sometimes makes us laugh)
Can’t and can’t have are used to show that there is no possibility. (See certainty below)
Must have is the past form of must when it is used to express certainty.
Must and must have express stronger certainty than will and will have.
Can’t and can’t have express stronger certainty than won’t and won’t have.
Had to is the past form of must when it is used to express obligation.
Don’t have to/don’t need to/needn’t (BrE) are used to show that there is no obligation. Must not is used to show that there is an obligation not to do something.
The contracted forms needn’t and mustn’t are common in British English but rarely used in American English.
The contracted form oughtn’t is common in British English but rarely used in American English.
In this meaning, should and ought to are not as strong as will and must (see “Certainty” above).
From Longman’s Dictionary of Contemporary English, New Edition, 1991, pp 669-671.
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