Troubles With Number
From Handling Of Words part of The Complete Plain Words by Sir Gowers(1954)

The rule that a singular subject requires a singular verb, and a plural subject a plural verb, is an easy one to remember and generally to observe. But it has its difficulties.

(1) Collective words.
In using collective words or nouns of multitude (Department, Parliament, Government, Committee and the like), ought we to say "the Government have decided" or "the Government has decided"; "the Committee are meeting" or "the Committee is meeting"? There is no rule; either a singular or plural verb may be used. The plural is more suitable when the emphasis is on the individual members, and the singular when it is on the body as a whole. "A committee was appointed to consider this subject"; "the committee were unable to agree". Sometimes the need to use a pronoun settles the question. We cannot say "The committee differed among itself", nor, without risk of misunderstanding, "the committee on whom I sat". But the number ought not to be varied in the same document without good cause. Accidentally changing it is a common form of carelessness:

The firm has given an undertaking that in the event of their having to restrict production....
The industry is capable of supplying all home requirements and have in fact been exporting.
It will be for each committee to determine in the light of its responsibilities how far it is necessary to make all these appointments, and no appointment should be made unless the committee are fully satisfied of the need.

Conversely a subject plural in form may be given a singular verb if it signifies a single entity such as a country (the United States has agreed) or an organisation (the United Nations has resolved) or a measure (six miles is not too far; twelve months is a long time to wait).

(2) Words linked by and.
To the elementary rule that two singular nouns linked by and should be given a plural verb justifiable exceptions can be found where the linked words form a single idea. The stock example is Kipling's "The tumult and the shouting dies"; "the tumult and the shouting", it is explained, are equivalent to "the tumultuous shouting". But die would not have rhymed with sacrifice. Rhyming poets must be allowed some licence.

Perhaps these official examples might be justified in the same way:

Duration and charge was advised at the conclusion of the call.
Your desire and need for a telephone service is fully appreciated.

It might be argued that "duration and charge was equivalent to "the appropriate charge for that duration", and that "your desire and need" was equivalent to "the desire arising from your need". But it is safer to observe the rule, and to leave these questionable experiments to the poets.

Other instances of singular verbs with subjects linked by and cannot be so easily explained away. They are frequent when the verb comes first. Shakespeare has them ("Is Bushy, Green and the Earl of Wiltshire dead? ") and so have the translators of the Bible ("Thine is the kingdom, the power and the glory"). If we may never attribute mere carelessness to great writers, we must explain these by saying that the singular verb is more vivid, and should be understood as repeated with each noun — "Is Bushy, (is) Green and (is) the Earl of Wiltshire dead?" Those who like to have everything tidy may get some satisfaction from this, but the writer of official English should forget about these refinements. He should stick to the simple rule.

(3) Words linked by with.
If the subject is singular the verb should be singular. "The Secretary of State together with the Under-Secretary is coming".

(4) Alternative subjects.
Either and neither must always have a singular verb unless one of the alternative subjects is a plural word. It is a very common error to write such sentences as:

I am unable to trace that either of the items have been paid.
Neither knowledge nor skill are needed.

(5) When each is the subject of a sentence the verb is singular and so is any pronoun:

Each has a room to himself.

When a plural noun or pronoun is the subject, with each in apposition, the verb is plural:

They have a room each.

(6) Attraction.
The verb must agree with the subject, and not allow itself to be attracted into the number of the complement. Modern grammarians will not pass "the wages of sin is death". The safe rule for the ordinary writer in sentences such as this is to regard what precedes the verb as the subject and what follows it as the complement, and so to write "the wages of sin are death" and "death is the wages of sin".

A verb some way from its subject is sometimes lured away from its proper number by a noun closer to it, as in:

We regret that assurances given us twelve months ago that a sufficient supply of suitable local labour would be available to meet our requirements has not been fulfilled.
So far as the heating of buildings in permanent Government occupation are concerned...

Sometimes the weight of a plural pushes the verb into the wrong number, even though they are not next to one another:

Thousands of pounds' worth of damage have been done to the apple crop.

In these sentences has, are and have are blunders. So is the common attraction of the verb into the plural when the subject is either or neither in such sentences as "Neither of the questions have been answered" or "Either of the questions were embarrassing". But in one or two exceptional instances the force of this attraction has conquered the grammarians. With the phrase more than one the pull of one is so strong that the singular is always used (e.g. "more than one question was asked"), and owing to the pull of the plural in such a sentence as "none of the questions were answered" none has come to be used indifferently with a singular or a plural verb. Conversely, owing to the pull of the singular a in the expression many a, it always takes a singular verb. "There's many a slip twixt cup and lip" is idiomatic English.

(7) It is a common slip to write there is or there was where a plural subject requires there are or there were.

There was available one large room and three small ones.

Was should be were.

It is true that Ophelia said "there is pansies". But she was not herself at the time.

(8) Certain nouns are sometimes puzzling.
Agenda, though in form plural, has been admitted to the language as a singular word. Nobody would say "the agenda for Monday's meeting have not yet reached me". If a word is needed for one of the components of the agenda, say "item No. so-and-so of the agenda", not "agendum No. so-and-so", which would be the extreme of pedantry. If one is wanted for the plural of the word itself it must be agendas or agenda papers.

Data, unlike agenda, remains the plural word that it is in Latin.

Unless firm data is available at an early date....

This is wrong. Is should be are.

If a singular is wanted, it is usually one of the data, not datum. The ordinary meaning of datum is:

Any position or element in relation to which others are determined: chiefly in the phrases: datum point, a point assumed or used as a basis of reckoning, adjustment or the like — datum line a horizontal line from which heights and depths of points are reckoned, as in a railroad plan. .. . (Webster.)

Means in the sense of "means to an end" is a curious word; it may be treated either as singular or as plural. Supposing, for instance, that you wanted to say that means had been sought to do something, you may if you choose treat the word as singular and say "a means was sought" or "every means was sought". Or you may treat it as plural and say "all means were sought". Or again, if you use just the word means without any word such as a or every or all to show its number, you may give it a singular or plural verb as you wish: you may say either "means was sought" or "means were sought"; both are idiomatic. Perhaps on the whole it is best to say "a method (or way) was sought" if there was only one, and "means were sought" if there was more than one.

Means in the sense of monetary resources is always plural.

Number. Like other collective nouns number may take either a singular or a plural verb. Unlike most of them, it admits of a simple and logical rule. When all that it is doing is forming part of a composite plural subject, it should have a plural verb, as in:

A large number of people are coming today.

But when it is standing on its own legs as the subject it should have a singular verb, as in:

The number of people coming today is large.

The following are accordingly unidiomatic:

There is a number of applications, some of which were made before yours.
There is a large number of outstanding orders.

The true subjects are not "a number" and "a large number" but "a-number-of-applications" and "a-large-number-of-outstanding orders".

Of the following examples the first has a singular verb that should be plural and the second a plural verb that should be singular.

There was also a number of conferences calling themselves peace conferences which had no real interest in peace.
The number of casualties in H.M.S. Amethyst are thought to be about fifteen.

Those kind of things. The use of the plural these or those with the singular kind or sort is common in conversation, and instances of it could be found in good authors. But public opinion generally condemns it. As I have said, the phrase those kind of things, like different to, very pleased, drive slow, and the split infinitive used to be among the shibboleths by which it was supposed to be possible to distinguish those who were instructed in their mother-tongue from those who were not. Years ago Punch published a poem containing this verse:

Did you say those sort of things
Never seem to you to matter?
Gloomily the poet sings
Did you say those sort of things?
Frightened love would soon take wings
All his fondest hopes you'd shatter
Did you say "those sort of things
Never seem to you to matter"?

We have a better sense of values today. But even now it is as well to humour the purists by writing things of that kind.