If the subject of a sentence is the same as the object of the verb of the same sentence, a reflexive pronoun is used.
David played tennis with David.
Because the subject is David and the object is also david, the correct way of saying this is:
David played tennis with himself.
himself is the reflexive pronoun for David.
Common singular reflexive pronouns are:
myself - I like to eat by MYSELF
yourself Help YOURSELF
herself she hurt HERSELF
Himself He surprised HIMSELF
Common plural reflexive pronouns are:
ourselves We went by ourselves. (Notice the spelling)
yourselves Help yourselves.
themselves - They made that themselves.
Reflexive pronouns can mean the same as the pronoun OWN.
I like to eat by myself I like to eat on my own.
by himself on his own.
by themselves on their own.
These occur very often in FCE sentence transformations.