Can and may compared

§ 87. The use of can and may is parallel only in two meanings:
possibility due to circumstances and permission. In these mean-
ings, however, they are not always interchangeable for a number
of various reasons.

1) Thus in the meaning of possibility due to circumstances the
use of may is restricted only to affirmative sentences, whereas can
is found in all kinds of sentences.

May Can

He may find this book at the He can find this book at the

library. library.

Can he find this book at the

library?

He cannot find this book at the
library.

Their time reference is also different. May refers only to the
Present or future; the form might is used in past-time contexts
only in reported speech. Can (could) may refer to the present,
Past or future.

May Can

He may find the book at the He can find the book at the

library. library.

I said that he might find the He could find the book at the

book at the library. library yesterday.

He can find the book at the
library tomorrow.

Both could and might combined with the Perfect infinitive in-
dicate that the action was not carried out in the past.

e.g. He might have found the book at the library.
He could have found the book at the library.

It follows from the above that the sphere of application of can
in this meaning is wider than that of may.

2) When may and can express permission the difference be-
tween them is rather that of style than of meaning — may is more
formal than can which is characteristic of colloquial English.

Cf. May (might) I speak to you for a moment, professor?
Can (could) I have a cup of tea, Mother?

May in negative sentences expressing prohibition is uncommon.

Must

§ 88. The modal verb must has only one form. It is used in
present-time contexts with reference to the present or future and
in combination with the Perfect infinitive it refers to the past. In
past-time contexts this form is used only in reported speech, i.e.
the rules of the sequence of tenses are not observed with must.

§ 89. Must has the following meanings:

1) obligation (from the speaker's point of view),

e.g. You must talk to your daughter about her future.
Must he do it himself?

In different contexts must may acquire additional shades of
meaning, such as duty or necessity.

In this meaning must is found in affirmative and interrogative
sentences and followed only by the simple infinitive.