Learn Danish: Drengen faldt og slog knæet. - The boy fell and hurt his knee.

In Danish, when you talk about body parts or clothing and it is obvious who they belong to, you usually use the definite form (adding -et or -en) instead of a possessive pronoun like hans or sit. So, knæet literally means "the knee," but it translates naturally to "his knee" in English.

Doesn't the verb slog normally mean "hit"? Why is it translated as "hurt"?

You are right, at slå usually means "to hit." However, Danish uses this verb to describe injuring yourself by impact, like in a fall. While English says "hurt his knee," Danish literally says he "hit the knee." If he had just hurt himself generally without naming a body part, it would be slog sig.

What makes knæet end in -et rather than -en?

The word for knee is a neuter noun in Danish (et knæ). To make an et-word definite ("the knee"), you add the -et suffix to the end, giving you knæet.

What are the infinitive forms of the past tense verbs faldt and slog?

They come from the irregular verbs at falde (to fall) and at slå (to hit/hurt). Because they are irregular, they do not take the standard past tense endings like -ede or -te, but instead change their forms entirely to faldt and slog.