Which sentence contains a grammatical error?

The sentence that contains a grammatical error is the one stating, "They has forgotten to submit the report." The subject "They" is plural, which requires the verb to also be in the plural form. The correct form of the verb "has" to match the plural subject "They" should be "have." Therefore, the sentence should read, "They have forgotten to submit the report." This subject-verb agreement is crucial in English grammar, ensuring that the verb correctly reflects the number of the subject, whether singular or plural. The other sentences exhibit proper grammatical structure, where the verb forms agree with their respective subjects.