The difference between "many" and "many a...."

Question

Umesh, from India, asked about the difference between many and many a/an...

Answer

Many

The word many has two common functions:

A) It is often used as an adjective that describes a plural noun and tells us that there is a large number of that noun, as in these examples:

1. She worked hard for many years. 
2. They were one of the many, many families that came to watch the parade.

B) Many is also commonly used as a pronoun, to mean “many people or things,” as in these examples:

1. Some people will come to the meeting, but many [=many people] will not.
2. We were hoping to sell our old books, but many [=many books] were not in good condition.
3. I know some of the people here, but not very many.

Many a/an...

The fixed expression many a/an... is more formal than the single word many, and it is much less common. Many a/an... is used mainly in literary writing and newspapers. Like the adjective and pronoun many discussed above, many a/an... is used to indicate a large number of something. However, it takes a singular noun, which can be followed by a singular verb. Here are some examples:

1. It remained a mystery for many a year.  [=for many years]
2. I've been there many a time.  [=many times]
3. Many a politician has promised to make changes. [Politician and has are singular.]

I hope this helps!