Modals are a special group of helping verbs: can and could, will and would, shall and should, may and might, and must. They are a tricky lot, bringing a range of nuances to sentences.
Perhaps the most interesting feature of modals is the contrast between their epistemic and deontic uses. Epistemic uses of modals relate to the speakers’ beliefs and opinions about an action as being possible or necessary. Deontic uses relate to canons or principles and express what ought to be or is permitted, logically or ethically.
A third use is known as dynamic and refers to ability, as in “Marketa can speak fluent Czech.” Together dynamic and deontic modals characterize real-world obligations, possibilities, and abilities as opposed to ones based on a speaker’s beliefs.
Let’s look at some examples.
Must shows its epistemic sense when a speaker says something like “it must be raining” after seeing evidence of rain, like wet people or dripping umbrellas. Might and may can also be used epistemically, with weaker evidence, like cloudy skies.
Must is used deontically to indicate necessity or obligation: “You must follow the instructions exactly,” while deontic uses of may indicate permission as in “You may borrow my book.”
Can is most often used dynamically to indicate ability, but it is also used deontically to give or ask for permission or to make a command (“You can go now.”). Can can even be used epistemically as in “The bridge can be icy” where it indicated a belief based on experience or knowledge.
Could functions much like can:
She could speak fluent Czech. (past ability or belief about a current possibility)
You could go now. (giving permission or acknowledging an opportunity)
The bridge could be icy. (belief based on experience or knowledge)
Will and shall are somewhat marginal modal verbs, used primarily for future actions and states, but even they show epistemic and deontic uses. The first sentence below reports a belief while the second sentence asserts an obligation.
The pizza will be here soon. (prediction based on belief)
Owners shall maintain the premises. (obligation based on law or contract)
Should behaves similarly, expressing both beliefs and obligations:
The pizza should be here soon. (prediction based on belief)
You should wash your hands and avoid touching your face. (obligation based on good practice)
Modal would is unlikely to be used deontically. Instead, would often expresses past habitual actions or a conditional future, viewed as a situation of high probability:
She would always send postcards. (habitual or usual action)
I would visit, but I’m afraid to fly. (belief)
That ringing would be the doorbell. (belief)
The distinction between dynamic, epistemic, and deontic is one of the most puzzling pieces of the verb system. For me, the easiest way keep things straight is with the mnemonic ABC: for ability, belief, and canon.
So when you encounter a modal, ask how it is being used. Is it A, B, or C?