[FionaVB]

I love reading Susan's replies to questions like this. She's such a mine of linguistic diamonds, and it's always explained so fully and clearly. Thank you Susan.
In terms of teaching the 3-P-S 's' and the plural 's', I tell my learners that one grammatical 's' in a sentence is enough. (Now I see that that could be confusing in a sentence like "dogs hate cats"!) But it works for "Cats hate water" and "My cat hates water." EITHER the noun OR the verb should have an 's'. At an elementary/young learners level, that can be a rule of thumb that will get them by until things get more complicated.