彼はズボンをアイロンがけしていた。

he; him; his; boyfriend

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

trousers; pants

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.

iron (for pressing clothes); hair iron; hair tong; curling tong

indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and

to change into; to convert to; to transform; to be reduced; to influence; to improve (someone)