One, still fiercely debated, is that language acquisition has an innate, genetic basis. Another is that prescriptive grammar, the rules of the copy editor and English teacher, are in large part arbitrary, will change, and in some cases ought to be flouted.
Some constructions are often the result of superstition more than grammatical analysis.
Also, I argue that language is not the same as thought, that we don't think in English but use it to express our thoughts to others. We use language as a kind of scratch pad, just as when you do mathematical calculations, and you've got to store the intermediate products on bits of paper. The fact that you can take away something more abstract than the verbatim sentences shows that there must be some medium of the mind. Talk about the distinctive nature of English. English probably has the largest vocabulary of any language, largely because it's a language of immigrants.