少年がその太鼓を叩いていた。

boy; juvenile; young boy; youth; lad

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

that (something or someone distant from the speaker, close to the listener; actions of the listener, or ideas expressed or understood by the listener); the; um...; er...; uh...

drum

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)