Grammar is the way we arrange words to make proper sentences. It is a set of rules, rules to make the right sentence. Grammar can help you to learn a language more quickly and more efficiently. When you understand the grammar of a language, you can understand many things yourself, without having to ask a teacher or look in a book.
The subject-verb agreement simply means the subject and verb must agree in number. This means both need to be singular or both need to be plural.
Here are some examples of subject-verb agreement (the subject is bolded and the verb underlined):
My dog always growls at the postal carrier.
Basketballs roll across the floor.
I don't understand the assignment.
These clothes are too small for me.
Peter doesn't like vegetables.