Learn Spanish (Latin America): El instructor, el cual suele observar el combate, es muy estricto. - The instructor, who usually observes the combat, is very strict.

In this non-defining relative clause (a clause set off by commas that adds extra background information), you can use el cual, quien, or que. El cual is more formal and is often used in written Spanish to sound precise and elegant. Since we are referring to a person (el instructor), quien would also be a great option, while que is the most common choice in everyday spoken Spanish.

Would el cual change if the sentence were about a female instructor?

Yes, this relative pronoun must agree in gender and number with the noun it refers to. If the sentence were about la instructora, it would become la cual. For plural instructors, you would use los cuales or las cuales.

Could I drop the commas around el cual suele observar el combate?

No, you generally cannot use el cual as a subject if you drop the commas. Without commas, it becomes a defining clause (specifying exactly which instructor you mean out of several), and in that case, you are required to use que. The commas make it a non-defining clause, which allows you to use formal options like el cual.

How does suele translate to "usually" when it looks like a conjugated verb?