Incorrect use: My brother and sister is in the car.

Should be: My brother and sister are in the car.

Explanation: There are two subjects in the car so the verb must be plural. By turning "is" into "are," the reader understands that both the brother and sister are in the car. The word is implies that only one is in the car.

Incorrect use: Heather always play outside during recess.

Should be: Heather always plays outside during recess.

Explanation: Heather is the subject in this sentence. Heather is one person, making the subject singular. Because of subject verb agreement, the verb must be singular as well. By using "play" the sentence suggests that Heather is plural, by changing it to "plays," the singular form, the sentence now clarifies that Heather is one person.

Incorrect use: The family are on vacation for the next two weeks.

Should be: The family is on vacation for the next two weeks.

Explanation: Although a family is composed of multiple people, the sentence describes one family, making the subject singular. Therefore the verb must be singular as well.