誰か戸をたたいている。

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

door (esp. Japanese-style); entrance (to a home); narrows

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 strike; to clap; to knock; to dust; to beat; to play drums; to abuse; to flame (e.g. on the Internet); to insult; to use up money; to call; to invoke (e.g. a function)