Grammar mistakes when writing, or even speaking, make a negative impression on colleagues, clients and potential employers. Subject-verb agreement is one of the most common mistakes people make. However, if you use "either" or "ni/ni", the use of a singular or plural verb depends on the noun closest to them: in the case above, we use the singular "is" when the singular "boy" comes after "or", but the plural "are" when the plural "girl" plural is in the last position. The subject-verb correspondence is determined by the number of subjects. The number of the subject is its singularity or plurality. If the subject is singular, the verb must match and also be singular. If the subject is plural, the verb must match and be plural. Note: Verbs do not make their plural by adding an s as nouns do.