Modal verbs include: "can", "may", "will", "shall" and "must".
Here's an everyday example of why they are eccentric. Take this sentence:
"He must be crazy."
Grammatically this is odd because we expect: "he musts." (Compare: "He sounds crazy.") The same is true of the other modal verbs:
"She can swim. "(Not: "cans".) "It may rain." (Not: "mays".) "He will help you." (not: "wills".) Another grammatical peculiarity is that we use them to form questions (interrogative clauses) and negatives: "Can she swim?" (not: "Does she can swim?") "She can not (or "can't") swim." (not: "She doesn't can swim.") An even more exclusive feature is that they refuse to be used in any subordinate role. Most verbs allow us to use them like adjectives or nouns:
"She likes sport. Liking sport is important." Modal verbs will have none of this:
"She can swim." but not: "Canning swim is important."
Instead we have to say: "Being able to swim is important."
Now consider the power of modal verbs to change meanings. Their effect is related to truth and certainty. Look at how the meaning is affected by adding the various modal verbs. Take this non-modal sentence:
"He is at home - I just saw him there."
Watch what happens when the modals arrive:
"He must be at home - his bike is outside."
"He will be at home by now - he's always home by this time."
"He may be at home - but equally he may not."
The idea expressed changes from fact to inference to mere possibility. Further subtleties emerge when we change tense. For example, "may" is slightly more certain than its past tense "might":
"He may be at home - but I'm not sure."
"He might be at home - but I doubt it."
Modal verbs aren't the only words that express this kind of meaning, of course. Adverbs, adjectives and nouns can all achieve similar semantic effects, though each opens up a different range of grammatical possibilities: Modal verb: "He may be at home."
Adverb: "Maybe he's at home."
Adjective: "It's possible that he's at home."
Noun: "We're considering the possibility that he's at home."
So that's the shady world of the modal verbs uncovered.
An encounter with them by key stage 2 or KS3 students will help them to explore the way our language choices allow us to create subtle (and not so subtle) shifts in meaning - a vital part of the writer's tool kit.
Richard Hudson is professor of linguistics at University College, London Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk