Two easy-to-make word confusions in American Standard English are:
These are not grammar errors, but rather, problems of correct usage. It is a problem of using the wrong word. One hears these mistakes so often that, after a while, the wrong choice starts to “sound right” and the right choice sounds strange. Do not worry: These two are very easy to master.
May versus Can
“May” means “to be allowed” or “to have permission.” In a slightly different context, it can mean “might,” as in “I may go to the store tomorrow.” Context alone will tell you if the statement is talking about something that is permitted or something that is merely a possibility.
Virtually all “may/can” errors involve using “can” when the correct word is “may.” The error almost always occurs when asking for clearance or permission: “Can I go now?” For generations, parents have been responding, “Of course you can go, but the question is whether you may go!” Then you are expected to rephrase the question correctly. Why not save some trouble for everyone involved, and ask it correctly the first time?
Shall versus Will
Like “may/can,” “shall/will” is fairly straightforward. All you have to do is know what the rule is, and then follow it. Most careless or uneducated speakers use “will” for everything, as if “shall” did not exist in the language. It has come to the point that even the experts in grammar regard them as interchangeable. Chaucer used them as such. But in the 17th century, the tradition arose to give each word its own use. So the “shall/will” question is about a traditional usage, which seems sensible, as opposed to a hard and fast syntactical requirement. The correct use of “will” and “shall” identifies you as a person who has command of his mother tongue; so it is a good idea to follow this rule, even if some moderns regard it as a bit stuffy.
Here is the rule; it comes in two parts:
“Will” is used for the future tense for verbs in the second person (you singular/you plural) and in the third person (he/she/it/they).
“Shall” is used for the future tense for verbs in the first person only (I/we).
If you are just prognosticating, you should use “shall” for “I” or “we” and “will” for everyone else: “You will enjoy the play” and “We shall enjoy the play.” Likewise, “She will return tomorrow” and “I shall return tomorrow.”
Confusion arises when, as a speaker, you want to add extra force or impact to your statement. Instead of merely predicting what will happen (using the future tense), you want to make a firm or emphatic assertion, an affirmation. This can be interpreted as an imperative — that is, a command — either to yourself (using the first person) or to other people (using the second or third person). In this instance, part I of this rule is flipped onto its head: Use “will” in the first person, and “shall” in the second and third persons.
“She shall return tomorrow” is not simply a prediction; it is either a command or a statement made to indicate that “she” (whoever “she” is) has no choice in the matter, and must return, no matter what.
Likewise, “I will return tomorrow” is more than a statement of your intention; it implies that you are determined to return, or that you have no choice but to return.
To illustrate: “We shall overcome” is a statement that predicts something about the future. “We will overcome” expresses determination, or, in the alternative, that there is no other option: overcoming is inevitable.
Now, just when you think you have it, there is one more special usage that seems to swim upstream against the rule. If you want to ask someone “Do you want to dance?”, a refined way of putting the question is: “Shall we dance?” But “shall” is simply the future tense in the first person, and this seems like a veiled request. Why would it not be “Will we dance?” I don’t know. It seems like an exception to both parts of the rule. Just remember it as a special case, I guess.
If these subtleties of meaning are a little cloudy, just go with part I of the rule, and leave part II for later.