here's a trick for learning english
the -er ending denotes an actor, someone doing a thing
like a chopper chops, and a hanger hangs, so does an importer import, a barometer baromet, and a sister sist
you can also use er- as a prefix as well, to denote the opposite relationship ie. someone watching a thing being done - an errant is watching someone rant, "erratum" refers to observers of a ratum (singular form of "rats"), and an error is currently hearing something ror