What is subject-verb agreement?
Subjects and verbs must agree in number (singular or plural) in order to make a logical sentence.
In present tense (meaning it's happening in the present), subject-verb agreement looks like this:
Notice how the verb has the
Sentences without subject-verb agreement would be:
You see how that sounds funny? That's because the subject and verb don't agree - they're singular and plural in the same sentence. These sentences are incorrect.
If you need to know subject-verb agreement in the different tenses, this is a good article to read.