If the subject is singular the verb must be singular. If the subject is plural the verb must be plural.
- The child goes to school every day.
(Child is singular + go + es for he/she/it.)
- The children go to school every day.
(Children is plural + go. No -es because children is they.)
- Robin agrees with the plan.
- Jack and Jill agree with the plan.
- If people want to be good at something, they have to work hard.
- The cat is walking, but the dogs are sitting.