彼は倒れたときにひざを痛めた。

he; him; his; boyfriend

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

to fall; to collapse; to drop; to fall senseless; to be forced to bed (by illness, etc.); to die; to go bankrupt; to be ruined; to have a bad debt; to be defeated (in a game); to fall (of governments, dictators, etc.)

by the way; incidentally; sometimes; occasionally

knee; lap; knee and thigh (while sitting)

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to hurt; to injure; to cause pain; to harm; to damage; to spoil; to worry; to bother; to be grieved over; to afflict; to cause financial loss; to hurt one's pocket