A number of vs the number of

I'm going to let you into a little secret: proofreaders don't remember everything and sometimes we have to look things up. And we double-check a lot because people like to point their fingers in a superior way when a proofreader makes a mistake. Consequently, I take twice as long as anyone else does to send emails and texts (was that double check, double-check or doublecheck?).

That's written English, though. Spoken English is quite another thing. When I'm speaking I'm more worried about getting my idea across (before someone talks over the top of me) and less concerned with being absolutely and grammatically correct all the time. So, late one evening, after watching an episode of War and Peace and still recovering from the amputation scene at Borodino, my mind wasn't quite up to the casual question thrown at me by a family member. I chickened out and said 'I dunno, I'll look it up.'

Here's the question:

When I'm using 'a number of' do I use a singular or a plural verb?

 Well, you could add another question to that one.

When I'm using 'the number of' do I use a singular or a plural verb?

Did you see the subtle difference? 'A number of' and 'the number of'. Don't be fooled by how alike they look. We treat them differently.

A number of

A number of soldiers are in the medical tent.
(Not a number of soldiers is in the medical tent.)

The soldiers (more than one so it's a plural noun) are what we need the verb to agree with (match). For one soldier we would use the singular form of the verb ('a soldier is...'). For more than one soldier we use the plural form ('the soldiers are').

The number of

The number of soldiers in the medical tent is horrifying.
(Not the number of soldiers in the medical tent are horrifying.)

This time the noun we want the verb to agree with is 'the number of' and this is a singular noun. So we use the singular form of the verb ('the number of...is...).

Simple Rule

A number of needs a plural verb.

The number of needs a singular verb.

Handy tip:
If you are having trouble with what the singular form of a verb is for this situation then just use the verb that goes with 'he/she'.   he runs, she congratulates, he was, she has
For a plural form of the verb use the form that goes with 'they'.   they run, they congratulate, they were, they had