Theory:

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.