Which sentence is grammatically incorrect?

The sentence "The cats is playing in the yard" is grammatically incorrect due to the subject-verb agreement. In this case, "cats" is a plural noun, which requires the corresponding plural form of the verb "to be." The correct form should be "The cats are playing in the yard," where "are" matches the plural subject.

The other sentences are correct because they maintain proper subject-verb agreement. "The puppies are house trained" correctly uses "are" with the plural noun "puppies." Similarly, "He drank the juice" and "They walk to school every day" both correctly use singular and plural forms of the verb, respectively, matching their subjects appropriately.