Modal auxiliary verbs
These are: will, would, shall, should, can, could, may, might, and must.
The text below is cut n pasted (and edited). Originally located at: http://www.quickanddirtytips.com/education/grammar/modal-auxiliary-verbs
Modal auxiliary verbs are defective—yes, that’s the actual term, defective. It means they’re missing some forms. For example, they don’t have third-person singular present tense forms—or to put it more plainly, sentences like he cans, she mays, and it woulds are ungrammatical. They also don’t have infinitive forms, so even though it would make sense, a sentence such as They seem to should practice more is ungrammatical.
Another way in which modal auxiliaries differ from lexical verbs is that their past tense forms usually don’t show past time. In fact, you might not have even realized that some modal verbs are actually past-tense forms. Will, shall, can, and may are present-tense forms. The corresponding past-tense forms are would, should, could, and might. Must doesn’t have a separate past tense.
Modal Auxiliary Verb Can Show How Likely Something Is
Instead of showing past time, past-tense modals typically perform one of two other functions. One of these functions is called modal remoteness, which is a technical term for unlikelihood. This is what you get in conditional sentences such as “If I won the lottery, I could start a new business.” Even outside conditional sentences, past-tense modals show this kind of remoteness. For example, telling someone “She would help you” suggests that you just need to give her the word, whereas “She will help you” means it’s as good as done.
Modal Auxiliary Verb Can Show Politeness
In a more specific kind of modal remoteness, the past tense of modal auxiliaries can show politeness. If you’re a native English speaker, you may have a gut feeling that it’s more polite to ask someone, “Could you do me a favor?” than “Can you do me a favor?” It sounds a little less pushy. It shows that you’re not assuming the person is just naturally going to do you a favor, and in this way it conveys politeness.
Modal Auxiliary Verb Can Change to Match the Tense of Other Verbs
Aside from modal remoteness, the other function that modal past tenses perform is backshifting, or as it’s sometimes known, sequence of tenses. Suppose Squiggly says to Aardvark, “I may go skiing in November.” If Aardvark is talking to Fenster about Squiggly later on, he might say, “Squiggly said he might go skiing in November.” The modal verb may gets put into the past tense might,not to indicate past time or show modal remoteness, but just to match the past tense said. Lexical verbs can backshift, too. If Aardvark tells Squiggly, “You’re my second-best friend,” and Squiggly tells Fenster about it later, he might say, “Aardvark said I was his second-best friend,” using was just to match the past tense said.
There Are Other Modals Too (known as Marginal modals)
In addition to may, might, can, could, will, would, shall, should, and must, there are a few fringe members of the family of modal auxiliaries. One of them is ought, which is different from the others because it’s the only modal verb that takes an infinitive. So you can say, “We must go,” or “We should go, but if you use ought, it’s “We ought to go.” Even further out on the fringe are some archaic uses of need and dare, as in Silly people need not apply, and How dare you speak to me that way?
The text below is cut n pasted (and edited). Originally located at: http://www.quickanddirtytips.com/education/grammar/modal-auxiliary-verbs
Modal auxiliary verbs are defective—yes, that’s the actual term, defective. It means they’re missing some forms. For example, they don’t have third-person singular present tense forms—or to put it more plainly, sentences like he cans, she mays, and it woulds are ungrammatical. They also don’t have infinitive forms, so even though it would make sense, a sentence such as They seem to should practice more is ungrammatical.
Another way in which modal auxiliaries differ from lexical verbs is that their past tense forms usually don’t show past time. In fact, you might not have even realized that some modal verbs are actually past-tense forms. Will, shall, can, and may are present-tense forms. The corresponding past-tense forms are would, should, could, and might. Must doesn’t have a separate past tense.
Modal Auxiliary Verb Can Show How Likely Something Is
Instead of showing past time, past-tense modals typically perform one of two other functions. One of these functions is called modal remoteness, which is a technical term for unlikelihood. This is what you get in conditional sentences such as “If I won the lottery, I could start a new business.” Even outside conditional sentences, past-tense modals show this kind of remoteness. For example, telling someone “She would help you” suggests that you just need to give her the word, whereas “She will help you” means it’s as good as done.
Modal Auxiliary Verb Can Show Politeness
In a more specific kind of modal remoteness, the past tense of modal auxiliaries can show politeness. If you’re a native English speaker, you may have a gut feeling that it’s more polite to ask someone, “Could you do me a favor?” than “Can you do me a favor?” It sounds a little less pushy. It shows that you’re not assuming the person is just naturally going to do you a favor, and in this way it conveys politeness.
Modal Auxiliary Verb Can Change to Match the Tense of Other Verbs
Aside from modal remoteness, the other function that modal past tenses perform is backshifting, or as it’s sometimes known, sequence of tenses. Suppose Squiggly says to Aardvark, “I may go skiing in November.” If Aardvark is talking to Fenster about Squiggly later on, he might say, “Squiggly said he might go skiing in November.” The modal verb may gets put into the past tense might,not to indicate past time or show modal remoteness, but just to match the past tense said. Lexical verbs can backshift, too. If Aardvark tells Squiggly, “You’re my second-best friend,” and Squiggly tells Fenster about it later, he might say, “Aardvark said I was his second-best friend,” using was just to match the past tense said.
There Are Other Modals Too (known as Marginal modals)
In addition to may, might, can, could, will, would, shall, should, and must, there are a few fringe members of the family of modal auxiliaries. One of them is ought, which is different from the others because it’s the only modal verb that takes an infinitive. So you can say, “We must go,” or “We should go, but if you use ought, it’s “We ought to go.” Even further out on the fringe are some archaic uses of need and dare, as in Silly people need not apply, and How dare you speak to me that way?