誰かがドアをノックした。

who

indicates a question (sometimes rhetorical); or; whether or not; some- (e.g. something, someone); indicates doubt, uncertainty, etc.; emphatic prefix; suffix forming adjectives or adverbs; in that way

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

door (Western-style)

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.

knock; knocking; fungo hitting; hitting balls for fielding practice