a number of

In an exercise we were working on in class, a sentence began with A number of students. This caused confusion because a number indicates one, so it would seem that the verb should be has. However, the correct answer is A number of students have. The reason the plural verb form is used is that a number of is an idiom. The idiom, a number of, means several. With this in mind, we can go back to our sentence

A number of students (has, have) problems with pronunciation.

and substitute several for a number of.

Several students (has, have) problems with pronunciation.

With several, it is clearer that the correct verb choice is have.

Several students have problems with pronunciation.

A number of students have problems with pronunciation.