In theory, the number aspect of subject-verb agreement seems like a very easy task. Just make sure that if you have more than one subject, a plural subject, you use a plural verb. All you need to do is decide between "one" versus "more than one" and you have your answer.
But some nouns do not allow themselves to be counted (non-countable nouns), while others function as plurals even though there is really only one thing. One of my favorite examples is "homework." Back in college, I had a German friend who always said "homeworks," which works great in German (Hausaufgaben) but is rather jarring to the English ear. Homework, it seems, always functions as a singular subject, no matter how much of it there is. In this respect, homework is like weather, water, sand, and coffee. You can fill an entire beach with sand or a large pot with coffee, but they are generally still singular in usage.
This rule seems simple enough. But what about the "sands of time" or the "coffees of South America" or Keats's "moving waters at their priest-like task"? This is where the confusion begins. Many nouns have both countable and non-countable senses, so it isn't enough to say that one can never use "sands" or "waters."
However, some noun-countable nouns do seem more resistant to pluralization than others. "Homework" would be one. Yet other nouns, such as "experience," seem perfectly at home in either the non-countable or countable camps. "In my experience" expresses an idea almost identical to "from my experiences I can tell you." How can we know how much or how many experience(s) we have had? Here is a list of certain "problem" plurals a best-guess recommendation for usage. As always, I welcome any additions, suggestions, or amendments to my list.
experience: a toss-up. Just try to consider whether you want to discuss your experience as a collective whole or individual experiences over time.
homework: non-countable. It is possible to have homeworks (my spell-checker flags this word by the way), but usually these are expressed in phrases like "pieces of homework."
time: both. As an abstract concept, like in "time flies," it is non-countable. As shorthand for various points in time, it is countable. An example would be a singer discussing his performance "times" or a track runner referring to the "times" she ran in a previous meet.
water: generally non-countable. I have only really seen "waters" in poetic or proverbial contexts (the waters of the ocean blue). For general usage, stick with "water," and if you must pluralize, go with phrases such as "bottles of water."
weather: always non-countable. I can imagine saying "the weathers have been terrible," but I have never heard or read such a thing. Stick with the singular.
In closing I just want to point out that there are also certain nouns that are always expressed in the plural even though they represent a single object. The three most common are "glasses," "scissors," and "jeans/pants/shorts." Just as my German friend had a little trouble with "homeworks," I too had some trouble in Germany when I kept referring to my jeans in the plural. If it's a pair of jeans, then that means there are two, right?