A practical english grammar - Vyssaja skola 1978
The verb BE
Sound symbols
This verb is quite exceptional in English, both in its forms and in its grammar. Much of its behavior has to be stated in special rules, which do not apply to other verbs. First of all, be has eight forms; no other verb in English has more than five. See Table 2-2:
Table 2-2: Forms of be
a That is, with the pronouns indicated or with any noun construction that can replace them.
b Were is used with all subjects in unreal conditions; see Chapter 18.
The verb be usually connects its subject with a complement, which may be a noun, adjective, or adverb. The meaning of be is that the subject and the complement are the same thing or person (That man is my father), that the subject is a member of the class of things named by the complement (My brother is a doctor), or that the complement describes or locates the subject (She is pretty; They are in the house). It is only rarely that be means simply “exist,” as in the sentence Those things can’t be!
The extremely important use of be as an auxiliary verb will be discussed later.
Contractions of BE with subject pronouns. The forms of be are usually spoken in the form of contractions after the subject pronouns:
The contractions are used only when another word follows in the phrase. If the form of be is final, as in its use as echo verb (Chapter 6), then the contractions can not be used.
Are you his brother? Yes, I am.
Is is usually weakened in speech to /z/, /s/, or /əz/, and are is usually weakened to /r/ or /ər/, depending on the sound of the preceding word. These reduced forms are not always indicated in writing, except that in informal writing the spelling’s is used for the /z/ and /s/ reductions of is.
John’s here.
My aunt’s over there.