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1. Each is the subject here, so if you were to take out the (of my classes), the sentence would be "Each is 50 minutes long.". Don't be fooled by the plural of the "classes" in the prepositional phrase. This first sentence is correct. Each is singular and so is the verb "is".
2. Several is the subject here which requires plural verb (are). This sentence is correct.
3. "request" is the subject, "has been approved" is the singular verb. This sentence is correct.
4. "professors" is the subject (plural) and "have tenure" is the plural verb. This sentence is correct.
5. "we" is the subject here (plural) but because of the word "either" the verb must be singular. This sentence is NOT correct. Here is the correct sentence: We believe either Mary or Sue is responsible for the theft. In other words, Mary and Sue could not BOTH have committed the theft.
6. "jury" is the subject. Jury is singular but it describes a group of people, so this may be tricky but since "jury" is singular (no matter what the meaning), verb must be singular as well "is going to return" is correct here. This sentence is correct.
7. "the boy, his sisters" are the subjects. Because he has more than one sister, there are at least 3 children not going to school so that would make this sentence subject plural. "are going" is the plural form. This sentence is correct.
Please let me know if you need any clarification. I'm always happy to answer your questions.
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