In essence, subject-verb agreement is making sure that your subject and your verb are either both plural or both singular. This is could be something that you’ve lost points for in your papers because it can seem right to write anything that sounds normal when you say it. For example, if you talk about someone and how they just had a vacation, that’s actually incorrect! That’s because “someone” is singular and “they” is plural. It may sound alright, but it is definitely wrong grammatically.
Now that you know what subject-verb agreement is, let’s work on finding it in your paper. There are only three simple steps!
So for example, in the sentence “The teacher, in addition to his students, are nervous about the exam,” “teacher” is the subject and “are” is the verb. Get rid of all the other words in the sentence and just say the subject followed by the verb. You should notice that there is incorrect subject-verb agreement. You want to say, "A teacher is," not "a teacher are."
Thus, the correct sentence should say, “The teacher, in addition to his students, is nervous about the exam.” To correct these issues, you can change either a plural noun or verb to be singular, or a singular noun or verb to be plural.
NOTE: There are some situations that are a little more difficult to understand, especially when it comes to discussing two subjects in a sentence.
For even trickier subject-verb agreement problems, check out OWL Purdue.