In the English language, 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).
The modal verbs in English are as follows, paired as present and preterite forms:
* shall and should
* will and would
* may and might
* can and could
* mote (Archaic) and must
The following are not modal verbs but may be used for a similar purpose:
* ought to and had better
* used to
* dare and need
* do
* going to
* have to
Although historically referring to past time, the preterite forms have come to be used in many cases with no such meaning.
Syntax
If a verb is preceded by multiple auxiliary verbs including a modal, as in "it could have been eaten," the modal will always appear before the other auxiliary verbs. A verb or auxiliary verb following a modal always appears in its basic form (for example, "could have gone" instead of "could had gone").
Past time use of preterite forms
Preterite forms may be used when referring to situations seen from the perspective of an earlier time. For example, would is originally the past tense of will, and it can still be used in that sense. The statement "People think that we will all be driving hovercars by the year 2000", in the context of the 1960s, can be represented in the present by replacing the verbs in italics by the appropriate preterite forms: "In the 1960s, people thought we would all be driving hovercars by the year 2000." Likewise, "I can do that" may become "I could do that when I was younger, but not anymore."
Conditionals
The preterite forms can also be used in the apodosis in the conditional mood, such as in counterfactual conditionals: "If they had wanted to do it, they would have done it by now." "If you bought a bus pass, you could catch as many buses as you liked without worrying about the cost of the fares." "If he were more polite, he might be better liked."
There is not always an explicit protasis ("if" clause) in this use: "Someone who likes red and hates yellow would probably prefer strawberries to bananas" means the same as "If someone who liked red and hated yellow were offered a choice of fruit, he or she would probably prefer strawberries to bananas." "I could help you with your work" gives a more tentative sense of ability to help than, say, "I can help you with your work" would. The implied protasis could, depending on the context, be along the lines of "If I wanted to".
Shall and will
Shall is used in many of the same senses as will, though not all dialects use shall productively, and those that use both shall and will generally draw a distinction (though different dialects tend to draw different distinctions). In standard, perhaps old-fashioned English, shall in the first person, singular or plural, indicates mere futurity, but in other persons shows an order, command or prophecy: "Cinderella, you shall go to the ball!" It is, therefore, impossible to make shall questions in these persons. Shall we? makes sense, shall you? does not.
Shall derives from a main verb meaning to owe, and in dialects that use both shall and will, it is often used in instances where an obligation, rather than an intention, is expressed.
Shall is also used in legal and engineering language to write firm laws and specifications as in these examples: "Those convicted of violating this law shall be imprisoned for a term of not less than three years nor more than seven years," and "The electronics assembly shall be able to operate within its specifications over a temperature range of 0 degrees Celsius to 70 degrees Celsius."
Should
Should is commonly used, even in dialects where shall is not. The negation is "should not" (or the contraction "shouldn't").
Should can describe an ideal behaviour or occurrence and imparts a normative meaning to the sentence; for example, "You should never lie" means roughly, "If you always behaved perfectly, you would never lie"; and "If this works, you should not feel a thing" means roughly, "I hope this will work. If it does, you will not feel a thing." In dialects that use shall commonly, however, this restriction does not apply; for example, a speaker of such a dialect might say, "If I failed that test, I think I should cry," meaning the same thing as, "If I failed that test, I think I would cry."
In some dialects, it is common to form the subjunctive mood by using should: "It is important that the law should be passed" (where other dialects would say, "It is important that the law be passed") or "If it should happen, we are prepared for it" (or "Should it happen, we are prepared for it"; where early Modern English would say, "If it happen, we are prepared for it," and many dialects of today would say, "If it happens, we are prepared for it").
Would
The contracted form of would is "'d". The negation is either "would not" or "wouldn't".
Would can be used in some forms that are viewed as more formal or polite. For example, "I would like a glass of water" compared with "I want a glass of water"; and "Would you get me a glass of water?" compared with the bare "Get me a glass of water."
"Would" can also be used for the imperfect tense. In the sentence "Back then, I would eat early and would walk to school...." "would" signifies not the conditional mood, but rather, repeated past actions of imperfect tense in English, and one must use care when translating to other languages.
May and might
May and might do not have common negative contractions (equivalents to shan't, won't, can't, couldn't etc.), although mightn't can occur in asking questions. ("Mightn't I come in if I took my muddy boots off?" as a reply to "Don't come in here! You'll get the floor dirty!")
Both forms can be used to express a present time possibility or uncertainty ("That may be."). Might and could can also be used in this sense with no past time meaning. Might and may would carry the same meaning in "John is not in the office today, and he could be sick."
May is also used to express irrelevance in spite of certain or likely truth: "He may be taller than I am, but he is certainly not stronger" may mean roughly, "While it is true that he is taller than I am, that does not make a difference, as he is certainly not stronger." (However, it may also mean, "I am not sure whether he is taller than I am, but I am sure that he is not stronger.") This is the meaning in the phrase "Be that as it may." Might can be used in this sense as well.
Might can be used in the first person to express that future actions are being considered. "I might go to the mall later" means that the speaker is thinking about going to the mall.
May or might can be used in a question to ask for permission. One who is saying "May I use your phone?” is asking for permission to use the phone of the person being spoken to. 'Can' or 'could' can be used instead, although formal American English prefers 'may'. In both cases the preterite form is viewed as more hesitant or polite.
Can and could
The negation of can is the single word "cannot", occasionally written as two words "can not" or the contraction "can't". The negation of could is "could not", or "couldn't".
Can is used to express ability. "I can speak English" means "I am able to speak English", or "I know how to speak English".
It is also used to express that some state of affairs is possible, without referring to the ability of a person to do something: "There can be a very strong rivalry between siblings" can have the same meaning as "There is sometimes a very strong rivalry between siblings".
Cannot and can't can be used to express beliefs about situations: "He cannot have left already; why would he want to get there so early?" expresses with less certainty the same proposition as "He has not left already" does.
Both can and could can be used to make requests: "Can you pass me the cheese?” means "Please pass me the cheese". Could can be used in the same way, and might be considered more polite.
Note that the form could is either preterite (past = was able to) or conditional (would be able to)
Must
Must has no corresponding preterite form. The negation is "must not" or "mustn't". An archaic variant is the word mote, as used in the expression "so mote it be".
Must and have to are used to express that something is obligatory ("He must leave"). It can be used to express a prohibition such as "You must not smoke in here", or a resolution such as "I mustn't make that mistake again".
There is a distinction between must and have to in the negative forms. In the sentence "You must not go", it is being expressed that it is obligatory for the person being spoken to not to go; whereas in the sentence "You do not have to go" it is being expressed that it is not obligatory for the person to go.
Have to can be used for an ongoing obligation, such as "he has to be careful".
Must and have to are used to express beliefs (the epistemic rather than deontic use), such as "It must be here somewhere" or "It has to be here somewhere", with the same meaning as "I believe that it very likely that it is here somewhere."
