-er/-or (a person who)
-er, -or
A person or thing that does something.
The visitor gave the teacher a basket of mangoes.
Teacher
A person who teaches.
Our English teacher reads a story every Friday.
Actor
A person who acts in plays or films.
Arjun wants to be an actor in Malayalam films.
Remember: No sure rule: -er is more common (singer, player). Learn the -or ones by heart: doctor, actor, visitor, inventor, sailor, author.