When to use -er -or -ar endings?

Answer 1

The -or and -ar endings for agents, as in professor and registrar, are generally found on words of Latin origin. The -er ending, as in bounder or talker, is found on native English words.

Answer 2

Use -er endings for verbs that indicate the actor performing the action (e.g., driver, teacher), -or endings for nouns indicating the doer of an action or the person in a position (e.g., director, professor), and -ar endings for verbs in the infinitive form in Spanish.