Verbs are almost always better in singular form because they imply (in English) both the present form and plural subjects - both good for verstality. Pluralized verbs connote a singular subject - and limit the possibile meanings for a verb standing by itself. And tenses and forms also have an effect on meaning - present participles are great because they imply action (shopping).
Nouns are interesting because most of the time it depends on what reality is. Uh oh! For example, there is only one "sun" so sun makes more sense than suns; the same with earth vs. earths. However, there are many, many more than one car, so cars is better than car. And so is hats to hat, etc.
Adjectives are better singular.
Adverbs are just ugly!
And some words are better singular even when we want them plural - money vs. monies or moneys :hehe:
-Damon (graduate degree in English!)