Which type of error does the sentence contain: She and her sister was going to the cinema?

The sentence "She and her sister was going to the cinema" contains a subject/verb agreement error. When the subject of a sentence consists of two nouns connected by "and," as in "She and her sister," it is treated as a plural subject. Therefore, the verb should align with the plural subject and use the correct form, which in this case is "were" instead of "was."

This type of agreement is essential in English grammar, ensuring that subjects and verbs match in number, helping to clarify the meaning of a sentence. In this instance, the plural subject indicates that the action is taking place involving both "she" and "her sister," thus requiring a plural verb form.