Warning: Proceed with Caution. Incorrect subject-verb agreement can summon dark forces that shall not be named.
The subject-verb agreement conundrum is a linguistic curse that afflicts the unwary. It can turn even the most skilled wordsmith into a mere novice.
When the subject is singular, the verb must follow suit. No exceptions.
One cat purrs on the mat.
Learn more about Rule 1 Proceed to Rule 2: Plural Subjects Demand Plural Verbs