Match the pronoun to the subject
The accusative set is mich, dich, sich, uns, euch, sich. In Du wäschst dich., the person washing and the person being washed are the same. Formal Sie also uses lowercase sich.
Wir waschen uns nach dem Sport.
We wash ourselves after sport.
Adding a body part changes the construction
Compare Du wäschst dich. with Du wäschst dir die Hände. In the second sentence, the hands are what you wash. This familiar body-care construction uses a dative person and a definite article with the body part.
Ich putze mir die Zähne.
I am brushing my teeth.
Different cases can have the same spelling
The contrast is visible in mich/mir and dich/dir. The forms sich, uns and euch look the same in accusative and dative. In Sie wäscht sich. and Sie wäscht sich die Hände., the spelling of the pronoun is unchanged but its role differs.
Ihr wascht euch das Gesicht.
You are washing your faces.
Some meanings need the reflexive expression
Learn sich freuen and sich fühlen as constructions. Ich freue mich auf den Urlaub. expresses anticipation. Das freut mich. means “That pleases me”; there, the subject is das, so mich is not reflexive.
Wie fühlst du dich heute?
How do you feel today?
Avoid the shortcut “body part always means dative”
A body-part noun does not automatically determine every case in a sentence. The verb and construction do. The body-care contrast is a useful starting pattern, not a universal definition of the dative.
Ich sehe meine Hände im Spiegel.
I can see my hands in the mirror.
Practise this pattern
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References
Original teaching prose and examples, informed by these grammar references. A2 placement is an editorial curriculum choice, not a CEFR certification.