There are a moon and stars in the sky.

In the given sentence, moon, stars, and sky are the general term used for things and places.

Hence moon, stars, and sky are the common nouns.

John works as a manager in the bank.

In the given sentence, John, manager and bank are general terms used for person and place.

Hence manager and bank are the common nouns.

My friend is playing basketball.

In the given sentence, friend and basketball are general terms used for person and thing.

Hence friend and basketball are the common nouns.