Breakdown of Hij knijpt me in de wang.
hij
he / it (referring to a 'de'-word)
in
in
me
me (unstressed form of mij)
knijpen
to pinch
Questions & Answers about Hij knijpt me in de wang.
Why does the sentence use de wang instead of mijn wang?
This is the possessive shift you are currently learning. In Dutch, when a physical action is done to someone's body part, it is more natural to use an object pronoun for the person (me) and a definite article for the body part (de wang), rather than a possessive pronoun like mijn.
Could I still say Hij knijpt mijn wang?
You could, and it is grammatically correct. However, Hij knijpt me in de wang is much more idiomatic. When describing physical contact with a body part, the structure using the object pronoun plus a preposition and the definite article is the standard way native speakers express it.
Why do we use the preposition in for a cheek? Wouldn't op make more sense?
The choice of preposition depends on the verb and the type of action. The verb knijpen (to pinch) usually pairs with in because you are grabbing and pressing into the tissue. So you pinch in de wang or in de arm, whereas you would pat someone op de rug (on the back).
Is me acting as a reflexive pronoun in this sentence?