Substituting Modal Verbs  

Modal auxiliary verbs express different shades of meaning; very often, they can be quite similar in how they are used, and it is sometimes unclear when it’s more appropriate to use one instead of another. Below, we’ll look at some comparisons between commonly confused modal verbs.

Indicating the future – Will / Shall

Will is the most common modal verb used to indicate one of the future tenses, as in:
We can also use shall in place of will to form the future tense, but only when the subject is in the first person (I or we). However, this makes the sentence very formal, and it is more commonly found in British English or in polite invitations. For instance:
  • “I shall be attending a dinner with the prime minister in April.”
  • Shall we dance, my dear?”

Requesting permission – Can / Could / May / Might

Each of these four modal verbs can be used to ask for permission, and each changes how formal the sentence is:
  • Can I open the window, please?”
  • Could I open the window, please?”
  • May I open the window, please?”
  • Might I open the window, please?”
In the above examples, can, could, may, and might are all used to request to permission do something.
Can is the least formal of the four, and some sticklers for grammatical etiquette will claim that it should not be used in this way at all; however, it is perfectly acceptable in informal conversation.
Could is more polite and a bit more formal than can. We can use this in most situations, except perhaps in very formal conversation.
May is more formal than either can or could, and it is commonly used as the standard modal verb to express or request permission.
Might is the most formal of them all; however, it can only be used to request permission (not to state that someone has permission), and it is not commonly used except in extremely formal circumstances.

Indicating ability – Can / Could

We often use can to indicate physical, mental, or functional ability in doing something. For example:
  • “I can speak three languages.” (I have the mental ability to do this.)
  • “He can swim very well.” (He has the physical ability to do this.)
Could is also used to indicate ability, but as the past tense of can. For example:
  • “He could speak three languages when he was four years old. (He had this ability by the time he was four years old.)
  • “She couldn’t ski until she was a teenager. (She did not have this ability until she was a teenager.)

Indicating possibility – May / Might

We use may and might to express a possibility. When describing a possible action, they generally indicate the same 50 percent chance of likelihood. For example:
  • “I might go to the movies today.”
  • “I may go to bed early this evening.”
May is regarded as being more formal in this use.
Both may and might are also used to indicate a possible outcome or set of circumstances. In this case, might tends to express less certainty or a lower likelihood than may, although the difference is slight. For example:
  • “We’ve tested thoroughly, but there might be some issues we’ve yet to discover.”
  • “He may have a chance of making a comeback in the polls.”

Making an offer – Can / May / Shall

Shall, may, and can can all be used to offer to do something for someone. For example:
  • Can I get the door for you?”
  • May I get the door for you?”
  • Shall I get the door for you?”
May is considered more formal and polite than can, although can is perfectly acceptable. Can and may, however, are both a little less direct than shall, which is used as more of a polite suggestion.

Making a request – Will / Can / Could / Would

All four of these modals can be used to make a request of someone, with differing degrees of politeness:
  • Will you get the door for me, please?” (most direct – least polite)
  • Can you get the door for me, please?” (slightly less direct – slightly more polite)
  • Could you get the door for me, please?” (less direct – more polite)
  • Would you get the door for me, please?” (least direct – most polite)
Could and would are the most polite modals to use for requests; however, the accompanying language we use (saying “please,” “if you don’t mind,” “if you could be so kind,” etc.) makes a bigger difference on the politeness of the request, no matter which modal verb is used.

Indicating an obligation – Must / Shall / Should / Will

When expressing an obligation to do something, we often use must, which is particularly direct and forceful. This might be found in a public notice, as in:
  • “Employees must wash their hands before returning to work.”
  • “Owners must clean up after their pets.”
Must can also be used in direct commands or directives, such as:
  • “You must finish your homework before you can go outside to play.”
  • “Students must put their names on their assignments, or they won’t be graded.”
We usually only find shall being used to express obligations in contracts or legal documents; it is used as a more polite and formal construction than must. For instance:
  • “The defendant shall pay the plaintiff $5,000 in damages.”
  • “The purchaser agrees that he or she shall forego any right to a refund after 90 days.”
When indicating obligation in more conversational English, we tend to use should instead, which is less formal than either shall or must. It is also less forceful than either, and it is used as more of a strong suggestion of what is best or most proper to do. For example:
  • “Students should hand in their assignments before Friday.”
  • “She should apologize for her behavior!”
  • “You should always pay your bills on time.”
We can technically use will in the same way as must, shall and should, but it is even more forceful than must and is less commonly used; it is generally reserved for strong commands or directives, as in:
  • “You will eat your vegetables or you won’t get any dessert!”
  • “They will agree to the terms of the lease or face eviction.”

1. In American English, when can we use shall instead of will to form the future tense?

2. Which of the following modal verbs is considered the most polite when requesting permission?

3. Which of the following is most commonly used to indicate obligation?

4. Which of the following modal verbs can be substituted for can in the following sentence?
“Excuse me, can you tell me where the nearest gas station is?”

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