When you use a relative clause, make sure your sentence also has a subject and a verb.

Tonight we were talking about relative clauses.

• relative clause 関係詞節 (definition from 英辞郎 on the web)

Here's a common mistake which even native speakers make:

× That red cabinet, which I can put books in. <- there's no main verb
○ That cabinet, which I can put books in, is red. <- the subject is "cabinet", and "is" is the main verb

Be careful not to use two subjects:

× Tadao Ando, who is an architect, he designed the New Tokyo Tower. <- two subjects, "Tadao Ando" and "he"
○ Tadao Ando, who is an architect, designed the New Tokyo Tower.

× The weather forecast, which says it will be cooler tomorrow.
× The weather forecast, which says it will be cooler tomorrow, it also says it will rain.
○ The weather forecast, which says it will be cooler tomorrow, also says it will rain.

I'm not looking forward to riding my bicycle in the rain. Hopefully it will just be light rain. Take care!