Introducing English Linguistics

Can
you give us the title of the book? 
(MICASE COL999MX040)
Could
you give us the title of the book?
By using can, the speaker is asking the addressee whether she can actual-
ly supply the name of the book: this is a question requiring a yes or no
answer. However, if could is substituted for can, the sentence becomes a
polite request for the book title. In other words, the speaker assumes that
the addressee knows the title of the book and is indirectly asking her for
the title. This use of could was extensively discussed in Chapter 3 in sec-
tions on indirect speech acts and politeness.
While only one modal can occur in a verb phrase at a time, more than
one primary auxiliary is possible. In examples below, two primary auxil-
iaries (have been) follow the modal auxiliary may.
Detectives said Mrs Page-Alucard may