Agent nouns. Verb in, doer out.

An agent noun is a noun built from a verb that names whoever, or whatever, performs the action. Teach becomes teacher. Drive becomes driver. German builds words this way constantly, and the pattern explains dozens of the suffix rules on this blog.

Already comfortable with the idea? Jump to German agent nouns in -er for the most common pattern, always der.

Verb in, doer out

Take a verb, add an ending, and the result names the person who does it. No translation needed, the logic is the same as English -er.

Verb	Agent noun	Meaning
lehren, to teach	derLehrer	teacher
fahren, to drive	derFahrer	driver
tanzen, to dance	derTänzer	dancer
schwimmen, to swim	derSchwimmer	swimmer

Not always a person

The "doer" doesn't have to be human. A machine or a tool that performs the action gets exactly the same treatment, just an agent noun for an object instead of a person.

Verb	Agent noun	Meaning
drucken, to print	derDrucker	printer
rechnen, to calculate	derRechner	computer
wecken, to wake (someone)	derWecker	alarm clock
öffnen, to open	derÖffner	opener

Masculine by default, -in adds the feminine

An agent noun built this way is masculine by default → derLehrer, derFahrer. To name a woman doing the same thing, add -in. The base word doesn't change, only the ending and the article do.

Masculine	Feminine	Meaning
derLehrer	dieLehrerin	teacher
derFahrer	dieFahrerin	driver
derTänzer	dieTänzerin	dancer

This -in ending is its own reliable rule. See German -in suffix for the umlaut changes and the -innen plural.

Not just -er

-er is the workhorse, but it isn't the only agent ending. Words borrowed from Latin and Greek often arrive with their own → -ent, -ant, -or, -ist. derStudent (one who studies), derMigrant (one who migrates), derAutor (one who writes), derPianist (one who plays piano). Same logic, same default: masculine, with -in for the feminine, dieStudentin, dieMigrantin, dieAutorin, diePianistin.