A/an is the indefinite article (singular).  Some/any is often used as the plural

You use a/an to talk about one particular person or thing, when the reader/listener does NOT know which one is meant.

My brother’s going out with a French girl

She lives in a nice big house

 

You use a/an to talk about one member of a group.

A doctor must like people (any doctor/all doctors.)

 

You can use a/an to signify which group/class they belong to.

I’m looking forward to being a grandfather.