English grammar rule A plural

The indefinite articles A or an modify singular nouns only. They do not modify plural nouns. However, when some numbers (hundred, dozen) modify nouns, indefinite articles also can replace one (one dozen, one hundred). Some informal expressions also use the construction many a, and in this case the noun must be singular (many a time).

Examples of 'A plural' rule

Incorrect	We have dozen eggs.
Correct	We have a dozen eggs.
Incorrect	Jimmy played a games with friends.
Correct	Jimmy played games with friends.
Correct	Jimmy played a game with friends.
Incorrect	Many a things have deteriorated lately.
Correct	Many things have deteriorated lately.
Correct	Many a thing has deteriorated lately.