Learn Spanish (Latin America): El tratado, el cual firmó el presidente, es importante. - The treaty, which the president signed, is important.

Yes! In everyday spoken Spanish, people would almost always say El tratado, que firmó el presidente, es importante. The phrase el cual means the exact same thing but is much more formal. You will mostly see it in writing, like news articles or official documents.

Why is it el cual and not lo cual?

El cual must match the gender and number of the noun it replaces. Here, it refers back to el tratado, which is masculine and singular. You only use lo cual when referring to an entire situation, action, or idea, rather than a specific noun.

Why is the word order firmó el presidente instead of el presidente firmó?

You can perfectly say el cual el presidente firmó. However, in Spanish relative clauses (the part of the sentence starting with a word like que or el cual), it is very common and natural to put the verb before the subject. It flows nicely and is heavily used in formal, written Spanish.

Do I have to include the commas around el cual firmó el presidente?