Use a reflexive pronoun when the SUBJECT and the OBJECT of your sentence are the same person, or thing.
In other words, the one doing the action, and the one receiving the action are the same.
- I can do it myself, thanks.
- You shouldn’t go by yourself. It’s not safe.
- Yes, he is getting married! He told me himself, so the rumour must must be true.
- Just stand in front of it, and the door will open by itself.
- They don’t have a lot of furniture, so they are going to move themselves.
- I don’t want you kids to go to the concert by yourselves.
- We thought we could figure it out ourselves, but in the end we needed to get a lawyer.