How well do you know the rules for subject-verb agreement?
How would you rate your command of English grammar? On a five-point scale, are you (1) very weak, (2) weak, (3) average, (4) strong or (5) exceptional?
Do you have a number or level in mind? Good.
Now let's see if your self-assessment is accurate. We'll use your knowledge of the rules of subject-verb agreement as a measure.
The good news about subject-verb agreement is that there is only one basic rule: Verbs must agree in number with their subjects. In other words, singular subjects take singular verbs, and plural subjects take plural verbs. Thus the term "agreement."
The bad news is that there are hundreds of exceptions and special cases, and those may vary depending on whether you're (or you are) using informal or formal English, American or British English, an English dialect or sub-dialect, etc.
For example, none and data are considered singular in formal usage, but those words can be either singular or plural in general usage. I'll use William Sabin's "The Gregg Reference Manual" as my source for rules and examples.
Ready?