Learn Russian: Он должен согласиться, потому что это очень выгодный проект. - He must agree because this is a very profitable project.

Согласиться is the perfective aspect. We use it here because we are talking about a single, specific action with a concrete result: him giving his agreement to this one specific project, rather than the general or repeated action of agreeing.

The word проект is masculine. Why do we use the neuter это instead of этот?

When это is used as a standalone pronoun meaning "this is" or "that is," it always stays in the neuter form, regardless of the gender of the noun that comes after it. If you wanted to say "this project" as a single phrase rather than "this is a project," you would use the masculine этот проект.

Does the word должен change its ending depending on who we are talking about?

Yes! Even though it translates to the English verb "must," должен is actually a short-form adjective in Russian. It must agree with the subject in gender and number. Because the subject is он (he), we use the masculine должен. For a female subject, it would be должна, and for plural, должны.

Since we are learning about зато (but on the other hand), could we use it here instead of потому что?