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The Oxford English Dictionary offers the following definitions of the modal verb can:
- To be able; to have the power, ability, or capacity.
- Expressing a possible contingency: To be allowed to, to be given permission to.
- Expressing possibility: To be permitted or enabled by the conditions of the case.
- Expressing an inclination in a conditional form.
Can expresses ability. For example:
- I can ride a bike. (Having the power, ability, or capacity, I am able to ride a bike.)
- You can drive a car. (Having the power, ability, or capacity, you are able to drive a car.)
- He can fly a plane. (Having the power, ability, or capacity, he is able to fly a plane.)
The modal verb can expresses permission. For example:
- You can see the movie. (You have permission to see the movie.)
- He can borrow my car. (He has permission to borrow my car.)
- Can they have some tomatoes? (Do they have permission to have some tomatoes?)
Can expresses possibility. For example:
- You can fly in a balloon with hot air. (Flying in a balloon is possible with hot air.)
- They can take the interstate to my house. (Their taking the interstate to my house is possible.)
- Chickens can fly. (It is possible for chickens to fly.)
The modal verb can expresses contingency. For example:
- If you can earn the money, then you may buy the dress. (Buying the dress is contingent on earning the money.)
- If the committee can provide the food, then I will provide the drinks. (My providing the drinks is contingent on the committee providing the food.)
- If you can have the report to me by Monday, that would be great. (It being great is contingent on your having the report to me by Monday.)
Can expresses requests. This fifth sense of can is closely related to the first and second senses of can. Requests made with can are often more polite than requests made through the imperative mood or commands. For example:
- Can you pass me the salt? (I am requesting that you pass me the salt.)
- Can you shut the door? (I am requesting that you shut the door.)
- Can you quiet down? (I am requesting that you quiet down.)
Note that there is overlap in the meanings of the modal verb can. For example, the contingency of If you can have the report to me by Monday, that would be great overlaps with the expression of a request: I am not simply stating that it would be great for you to get the report to me by Monday but am also requesting that you have the report to me by Monday.
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The Oxford English Dictionary offers the following definitions of the modal verb may:
- Expressing objective possibility, opportunity, or absence of prohibitive conditions; have the potentiality to, be at liberty to, be permitted by circumstances to.
- Expressing permission or sanction: be allowed (to do something) by authority, law, rule, morality, reason, etc.
- Expressing present subjective possibility, i.e. the admissibility of a supposition, in a direct or indirect statement.
- Used in a question, with the effect of rendering the tone less abrupt or pointed.
The modal verb may expresses possibility. For example:
- The rocket may explode at take off. (It is a possibility for the rocket to explode at take off.)
- I may take you up on that offer. (It is a possibility for me to take you up on that offer.)
- The bread may not turn out right. (It is a possibility that the bread will not turn out right.)
The modal verb may expresses probability. Note that the difference between possibility and probability is often expressed through adverbs. For example:
- The patient likely may never walk again. (It is probable that the patient will never walk again.)
- I may very well join you tonight. (It is probable that I will join you tonight.)
- Some guests may just not like coconut. (It is probable that some guests do not like coconut.)
May expresses permission and requests for permission. Note that the permission expressed by may is generally more polite than the permission expressed by can. For example:
- You may borrow my computer. (You have permission to borrow my computer.)
- May I borrow your computer? (Do I have permission to borrow your computer?)
- Drivers may not make U-turns at this intersection. (U-turns are not permissible at this intersection.)
Note that there is also overlap in the meanings of the modal verb may. For example, the difference between possibility and probability is extremely subtle and context dependent. Also note that the meanings of may overlap with the meanings of can with may generally considered more polite or formal.
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For a printable reference sheet of the definitions of the modal verbs can and may, please download English Modal Verbs Definitions Reference Sheet.
- Oxford English Dictionary; http://www.oed.com/