The number of birds in the tree was astonishing. A number of Nigerians speak French. The number of murders in New York City always increases in the summer. A number of trees leaves' turn yellow in the fall. "The number of..." sounds like a professor or newsperson talking. To say that "a number of students failed the midterm exam" sounds to me like perfectly spoken English! Both of these phrases are very useful when talking about a quantity or number that we are uncertain of.... therefore, we generalize an say "a number" instead of "a hundred", because to say a hundred or one hundred may be inaccurate.

Here's a simple answer: plural. Why? Because when you say 'a/the number of' you're referring to more than 1! E.G. A number of accidentS were reported today. Hope this helps.

A, An or The? When do we say "the dog" and when do we say "a dog"? (On this page we talk only about singular, countable nouns.) The and a/an are called "articles". We divide them into "definite" and "indefinite" like this: Articles Definite Indefinite the a, an We use "definite" to mean sure, certain. "Definite" is particular. We use "indefinite" to mean not sure, not certain. "Indefinite" is general. When we are talking about one thing in particular, we use the. When we are talking about one thing in general, we use a or an. Think of the sky at night. In the sky we see 1 moon and millions of stars. So normally we would say: I saw the moon last night. I saw a star last night. Look at these examples: the a, an The capital of France is Paris. I have found the book that I lost. Have you cleaned the car? There are six eggs in the fridge. Please switch off the TV when you finish. I was born in a town. John had an omelette for lunch. James Bond ordered a drink. We want to buy an umbrella. Have you got a pen? Of course, often we can use the or a/an for the same word. It depends on the situation, not the word. Look at these examples: We want to buy an umbrella. (Any umbrella, not a particular umbrella.) Where is the umbrella? (We already have an umbrella. We are looking for our umbrella, a particular umbrella.) http://www.englishclub.com/grammar/adjectives-determiners-the-a-an.htm