Ok so many of you know, my husband has mental retardation.
While helping my second grader with her homework, we were discussing adding an S or an 'ES' at the end of words to make plural.

Ayla had no trouble remembering which few words had NO plural change. Like sheep, cattle, deer, etc.

Issac on the other hand, ended up very frustrated when he tried to tell her that the plural for a sheep is 'sheeps' and she told him he was wrong. And we ended up in a long debate on WHY sheep doesn't get an s when plural.

I have NO idea how to explain this. What is the rule behind it IF there even is one.

I don't think there is a rule that makes consistent, logical sense. Words like sheep, deer, and fish are used both as singular and plural. Perhaps it has to do with the fact that these animals typically are gathered in groups (herds or schools) and so we think of them as a crowd and not as individuals. Why we don't treat other animals the same way I have no idea - we say "a flock of birds" rather than "a flock of bird." So it's not consistent.

But I would point out that "cattle" is plural only - you can't have just one cattle, and so you say "the cattle are..." not "the cattle is...". If you want to talk about an individual animal you call it a cow, or a steer, or whatever.

How about words that sound like they are plural but we treat as singular? For example consider "The United States." Back in the early days of the country people thought of the US as a collection of individual states, so would say "The United States are ..." But around the time of the Civil War as the federal government gained more power over the individual states the common usage became "The United States is..."

Another example: my British friends refer to companies as plural, whereas in the US we treat them as singular. Thus I might say "General Motors is a big company," and they would say "General Motors are a big company."

It's all very confusing. And that's why English is such a notoriously difficult language to get right.

Thank you so much. He learned Spanish first, his mother was an immigrant to the US and she didn't speak English yet so he didn't learn English until school. So I bet that's the problem. Spanish is so simple compared to English. He still doesn't get WHY, but he has accepted it and stopped arguing about it after I read your post to him :) thanks hon!