Several of my students confuse 3rd person singular and plural verbs. The book "Baby Bear Sees Blue" by Ashley Wolff lends itself well to addressing this issue.
I've created sentence strips in pairs that directly contrast 3rd person singular verbs and 3rd person plural verbs. The task is simple. After teaching the difference between the two, you read "Baby Bear Sees Blue" to the students.
Then present them with each contrasting pair and have them fill in the middle portion of the sentence with either the word "sees" (for sentences just about Baby Bear) or "see" for sentences about Baby Bear and Mama Bear.