How can I explain to a non-native why the plural of a noun isn't used adjectivally?

Question

How can I explain to a non-native speaker that while this is correct:

The felling of trees.

And this is also correct:

tree felling

This is not:

*trees felling

To a non-native it seems very reasonable that since many trees are being felled and given that the felling of trees uses the plural, the inverse form would also use the plural and become trees felling.

To a native speaker such as myself, the latter is very obviously wrong, but I couldn't figure out how to formalize this into a rule that a non-native can understand.

Answer 1

There is no simple explanation, and explanations given by linguists are not to date convincing. See, for example, Sneed's work, which builds on earlier work.

You can say that there is a rising trend, that seems to correlate with the increased use of nouns + noun combination (e. g., nowadays one is more likely a Cubs fan rather than a Cub fan). There is now covers band and not just cover band; jobs report, drugs dealer (and one thinks, drugs dealing).

Just spotted: birds conservation. This seems to be a British English usage, and such nouns + noun are used more often in BrE than AmE.

I found ESPN and the Washington Post both using Numbers Crunching, which seems novel compared to the singular option. You are free to google your favorite expression and see if anyone is (yet) using it in the plural.

Answer 2

Why do you want to explain it when there is no simple explanation. It should be sufficient to say that in compound nouns of the type noun + noun the first noun or subnoun is normally singular with a few exceptions.

But of course, one may think about why the singular is preferred. It might be a simple case of shortening: horse trainer is shorter and easier to speak than horses trainer.

Another reason might have been that in spoken language it might have been disturbing to decide whether girls class or girls' class was said, and that girl class was simply unambiguous.