Learn Swedish: Hennes knä blev svullet efter olyckan. - Her knee became swollen after the accident.

The base form of the adjective is svullen (swollen). Since it describes knä, which is an ett word, it needs a neuter ending. For adjectives ending in an unstressed -en, we drop the -n and replace it with a -t. This gives us svullet.

We are talking about a specific knee, so why is it Hennes knä instead of the definite form Hennes knät?

In Swedish, whenever you use a possessive pronoun like hennes (her), hans (his), or min (my), the noun that follows must be in its indefinite form. Even though the meaning is specific, the grammar requires the indefinite knä.

Could I use var (was) instead of blev (became) here?

You could say Hennes knä var svullet (Her knee was swollen), but using blev emphasizes the change of state. It shows that the knee got or became swollen as a direct reaction to the accident, which sounds much more natural in this context.

How would this sentence change if both of her knees became swollen?