I have a product that has high search numbers for both singular and plural. In the keywords meta tag should I just put one or the other or should I put single and plural for the product and all the long-tail keywords?
Or, you could just ignore it, like Google does. If a page / site already has a keywords tag (and it isn't a liability by being 2000 words, or using terms not appearing in the text), I'd tend to just leave it. If you don't have one, I wouldn't expend the effort to put it there