Relative clauses in Japanese allow you to modify a noun by describing an action related to it. This technique combines what would be two separate sentences into a single, more descriptive one. Typically, this is done using the plain past form (た) or the continuous form (ている) of a verb.

• 先週(せんしゅう)京都(きょうと)へ行(い)った友達(ともだち)。
  The friend who went to Kyoto last week.
• 私(わたし)が見(み)た映画(えいが)。
  The movie that I saw.
• 妹(いもうと)が聞(き)いている音楽(おんがく)。
  The music my sister is listening to.
• ベンチの上(うえ)で眠(ねむ)っている猫(ねこ)。
  A cat that is sleeping on the bench.

The conjugation rules for both る-Verbs and う-Verbs remain the same when creating a relative clause. This structure combines two separate ideas (e.g., 'This is a friend. He went to Kyoto.') into a single sentence. While English uses words like 'who', 'which', or 'that' to connect the clauses, Japanese achieves this by simply placing the modifying verb phrase directly before the noun.