1. linking singular and plural.
Very often the subject of the verb be is singular and the complement plural, or vice versa, and in these cases the verb should agree with the subject:

• Gustave is other animals as well —Julian Barnes, 1984

• These huge biographies are usually a mistake nowadays —N. Stone, 1985.

But when the subject is a collective noun, the verb may be in the plural, following the usual pattern with such nouns:

• Its prey are other small animals —David Attenborough, 1987.

When the subject is the relative pronoun what, the verb is singular:

• What I'm really interested in…is the objects in this house —New Yorker, 1986.

2. subjunctive forms.
The verb be has two residual subjunctive forms, be and were. Their use is rapidly disappearing, but there are several uses still left:
a) in an inverted construction replacing if or whether:

We would much prefer to support specific projects, be they in management schools or in university laboratoriesJournal of the Royal Society of Arts, 1986

/ Were this done, we would retain a separate Barwith skillTimes, 1986.
b) after if in hypothetical conditions:

If the truth be told, I never wanted to fly away with the sky-gods —J. M. Coetzee, 1977

/ If I were obliged to rough out a blueprint of the Church of the future, I would start with the need for good popular theology —Gerald Priestland, 1982. However, the past indicative form was is often used instead of were, especially in conversational style: I wouldn't tell the police if I was you —R. Hopcott 2002 [OEC].
c) in dependent clauses after verbs of advising and instructing such as demand, insist, suggest, etc:

The Admiralty insisted that the case be clarified —P. Wright, 1987

/ In order to broaden the ‘target audience’ of your newsletter…I might suggest that such material be written at a lower level of readability.Underground Grammarian, 1982; also after nouns and phrases of equivalent meaning: It is important in today's vote that the principle itself be acceptedTimes, 1985 / She demanded that they be clean and well-behaved —online essay, AmE 2005 [OEC].
d) in certain fixed expressions such as be that as it may, far be it from me, the powers that be, etc.
3. the case of the complement after be.
The monks of Rheims, as quoted by the grammarian Dean Alford in 1864, cried out when they saw the anathematised jackdaw: ‘That's him!’ and not ‘That's he!’. In both speech and writing, the type it's me/him/her/us/them is now virtually universal, except when a relative pronoun follows, as in

• It was he who would be waiting on the tow-path —P. D. James, 1986.

See further at cases 2.
4. reduced forms.
Am, is, and were are reduced to 'm, 's, and 're respectively after pronouns and nouns (I'm over here / She's just coming / We're late, are we?), except when the noun ends in a sibilant sound (☒The church's just up the road). Aren't, used for am not in the question form aren't I as well as are you/they not, is irregular; ain't is irregular and widely deplored (see ain't).
5. ellipsis of be.
Be is often omitted, especially in informal contexts, in cases such as They are sorry for what they did and anxious to make amends / We're leaving now and catching the 9.00 train. When be is used as an auxiliary verb and as a linking verb in the same sentence, it must be repeated because its role is different: The bill was overtaken by the 1964 election and its postponement was welcome.

Modern English usage. 2014.

(whether in fact or in imagination), , ,