Which error type applies to the sentence: Each of the books were great?

The sentence "Each of the books were great" contains a subject-verb agreement error. In this case, the word "each" is a singular subject, which means it requires a singular verb. The correct form should be "Each of the books was great," as "was" agrees in number with the singular subject "each." This highlights the importance of ensuring that subjects and verbs in a sentence match in terms of singular and plural forms, an essential principle in English grammar that helps maintain clarity and coherence in writing.