Learn Danish: Han skal have en operation i knæet. - He is going to have an operation on his knee.

In Danish, skal is often used to express a planned future event or arrangement, similar to "is going to" in English. While it can mean "must", the context of having a medical procedure implies a scheduled event rather than a strict obligation.

Why do we use the preposition i (in) instead of på (on) for an operation "on" the knee?

Danish prepositions often do not translate word-for-word. For medical procedures involving joints, internal organs, or deep tissue, Danish usually uses i (in) instead of på (on). You are literally having an operation "in" the knee.

Why doesn't the sentence say hans knæ or sit knæ for "his knee"?

When referring to body parts, Danish generally uses the definite form of the noun (here, the -et ending in knæet meaning "the knee") instead of a possessive pronoun like hans or sit (his), as long as it is completely obvious whose body part it is.

Is knæ an en-word or an et-word, and how does that affect the ending?

Knæ is a neuter word (an et-word). To make it definite ("the knee"), you add -et to the end, making it knæet. Because the word already ends in a vowel, the spelling simply adds the -et suffix directly.