In the given sentence: "We failed to notice the air hissing out of the air mattress, and now, three hours later, none remain."
The error with subject-verb agreement is in the verb "remain." The correct form should be "remains" to match the subject "none." The verb should agree with the subject in number. Since "none" is a singular indefinite pronoun meaning "not any," it requires a singular verb form, which is "remains" in this case.
Corrected sentence: "We failed to notice the air hissing out of the air mattress, and now, three hours later, none remains."
By correcting the verb to "remains," the subject-verb agreement is now accurate, and the sentence is grammatically correct.
