The second point is that one of the things that is known is that some grammatical elements are easier to learn than others. Take this list of English grammar points:
- present progressive –ing (Mommy running)
- plural –s (Two books)
- irregular past forms (Baby went)
- possessive 's (Daddy's hat)
- copula (Annie is happy)
- articles the and a
- regular past –ed (She walked)
- Third person singular simple present –s (She runs)
- Auxiliary be (He is coming)
I'd be hugely interested to know if anyone has tried researching such a table for cases other than English - looking at it, I thought immediately of Russian, which uses neither copula nor articles, but of course has numerous cases for nouns and distinguishes between transitive and intransitive verbs. Surely we could learn quite a lot about deep structure, including whether there is really much evidence for it in the first place, by comparing surveys like that across different (or indeed similar) languages?
Anyway, I shall continue the occasional browse of our language shelves.