According to this lesson, the verb must agree with the subject in
case
number
gender
tense

NUMBER
Since the question doesn’t specify the “lesson” being mentioned in this query. The most probable answer is number. Number because most lessons in subject-verb agreement concerns the singularity and plurality of the subject at hand. For example. The teacher, together with her students, is going to the classroom. Which is a singular sentence. Either the brothers is having fun. Another singular. Hence this is very important in grammar and as an English proficiency skill –basic.