彼はひざをいすにぶつけてしまった。

he; him; his; boyfriend

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

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.

chair; stool; post; office; position

at (place, time); in; on; during; to (direction, state); toward; into; for (purpose); because of (reason); for; with; by; from; as (i.e. in the role of); per; in; for; a (e.g. "once a month"); and; in addition to; if; although

to hit (e.g. one's head); to strike; to crash into; to throw (e.g. a ball, a question, a demand); to express; to vent (e.g. one's anger)