Please Help!

I need a rule of English grammar that has always caused people problems in English class (that is, a prescriptive rule)?

One common rule in English grammar that often confuses people is the proper use of subject-verb agreement. This rule states that the subject of a sentence must agree in number with the verb. In other words, a singular subject should take a singular verb, while a plural subject should take a plural verb.

To determine subject-verb agreement, you first need to identify the subject of the sentence. The subject is the word or group of words that performs the action or is being described in the sentence. Once you've identified the subject, you must ensure that the verb agrees with it in terms of singular or plural form.

For example:
- Incorrect: The cat on the table is cute.
- Correct: The cat on the table is cute.

In the incorrect sentence, the plural subject "cats" does not agree with the singular verb "is." To fix this error, you should use the singular verb "is" to match the singular subject "cat."

Another example:
- Incorrect: The girl with her friends were at the park.
- Correct: The girl with her friends was at the park.

In this case, the singular subject "girl" should be followed by the singular verb "was," rather than the plural verb "were."

To avoid subject-verb agreement errors, always make sure to identify the subject and then ensure that the verb matches in number (singular or plural) with that subject.