Use a reflexive pronoun when the action comes back to the same person: 'I hurt myself.' The subject and the object are the same.
Quick tip: Action comes back to the subject? Use a reflexive: myself, yourself, himself, herself, itself, ourselves, yourselves, themselves.
The forms: myself ... themselves
Each subject has its own reflexive: I → myself, you → yourself, he → himself, she → herself, it → itself, we → ourselves, you (plural) → yourselves, they → themselves.
I → myself, you → yourself
he → himself, she → herself
we → ourselves, they → themselves
Singular ends in -self; plural ends in -selves.
Use when the action comes back to the subject
When the subject does the action TO ITSELF, use a reflexive pronoun, not an object pronoun.
I cut myself while cooking.
She taught herself to paint.
They enjoyed themselves at the party.
Be careful! Don't hurt yourself.
Reflexive vs object pronoun
Same person → reflexive (myself). Different person → object pronoun (me, her, them).
She saw herself in the mirror. (same person)
She saw her across the room. (different person)
We helped ourselves to cake. (us)
We helped them with the bags. (other people)
Common mistakes
Not "I hurt me." — say "I hurt myself.". Same person → reflexive, not the object pronoun.
Not "They enjoyed theirselves." — say "They enjoyed themselves.". It's 'themselves', never 'theirselves'.
Not "He hurt hisself." — say "He hurt himself.". It's 'himself', never 'hisself'.
Practise it
Does the action come back to the same person? Then use a reflexive: 'She did it herself.'