I searched for open source English grammar checkers, naively thinking that they would work by fully parsing English, but most of them only do pattern matching to look for common errors - like LanguageTool, used by OpenOffice. They are peephole optimizers, not complete parsers.
Except for Link Grammar, used by AbiWord. This one does parse English!
Even though it doesn't answer my original question (it thinks that "explain me something" is okay), it is quite cool.
proclus: no offense man, but your photograph isn't that interesting. Do you really need to post a link to it so many times?