1. Subject–verb agreement errors
One basic rule of English grammar is that the subject (the one performing the action) must agree in number with the verb (the action or state of being). For example, in the sentence "Matt plays the guitar," both Matt and plays are singular, so this subject and verb agree. However, most sentences, especially in academic writing, aren 't so straightforward. Descriptive phrases can get in the way, making it difficult to determine if the subjects and verbs agree. When this happens, eliminate all intervening information to get to the meat of the sentence.

Incorrect: The girl with the black and white puppies have a ball.
Because puppies is right before have, this bad grammar example is easy to overlook. Ask yourself who the sentence is about (the girl), and eliminate the rest:

Correct: The girl has a ball.
2. Pronoun–antecedent agreement errors
Like subjects and verbs, pronouns must agree with their antecedents, the nouns they replace. They must agree in both number and gender. Typically, this is easy, as in the following example:

Correct: Yolanda has her notebook.
However, with certain words, it is more difficult to determine whether they are singular or plural. For instance, indefinite pronouns (such as someone, anyone, few, none, or everyone) confuse many English speakers, as in this bad grammar example:

Incorrect: Everyone needs to bring their pencil.
Here, everyone is singular, so the pronoun before pencil must be as well. It would