Verb Before a Noun: 買う, 買った, 買わない or 読んでいる

Japanese has no relative pronoun like English who, which or that. To describe a noun, put a whole clause in front of it, and end that clause in the plain form, never the ます form: 昨日買った本 (the book I bought yesterday), not *昨日買いました本.

• Future or habit: 買う, 食べる, 来る
• Past: 買った, 読んだ, 来た
• Negative: 買わない, 来ない
• Past negative: 買わなかった, 食べなかった
• Ongoing: 読んでいる (casual: 読んでる)

Pick the form from the clues inside the clause: 明日 or 今度 point to the dictionary form, 昨日 or 先月 to the past, 全然 and 誰も need a negative, and 今 for something still going on needs ている. Politeness appears only once, at the very end of the sentence (…です, …ました).