I think that you need to study the various pronouns -- Subject, Direct / Indirect object, Reflexive.
Your query relates to reflexive pronouns which match the subject "person" of which there are three -- I, you, him or her or it. singular and then the corresponding plural versions.
I recommend that you take each grammar element - understand, practise, test.
In time you will become familiar with the various aspects of the grammar and will be able to progress.
Here are two links to familiarise yourself with reflexive verbs and their pronouns:
Reflexive verb - se laver - Présent
Reflexive pronouns - glossary
I hope this is helpful.
Bonne journée !
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