Learn French: Je me suis fait mal au genou.

The French expression for 'to hurt oneself' is se faire mal. Because it is a reflexive verb (an action done to yourself), it must use être as its helping verb in the passé composé. That gives us je me suis fait.

Why is it au genou and not 'à mon genou'?

When talking about body parts, French generally uses definite articles (le, la, les) instead of possessives like 'my' (mon). The reflexive pronoun me already tells us whose knee is hurting! Since genou is masculine, à + le contracts into au.

If a woman is speaking, would she add an 'e' to make it je me suis faite mal?

No, the past participle remains fait. While reflexive verbs usually agree with the subject in the past tense, they do not when the reflexive pronoun is an indirect object. Because the underlying phrase is faire mal à quelqu'un (to do harm to someone), the me acts as an indirect object, meaning there is no agreement on fait.

How would I say this if I hurt both of my knees?