"Yet every child does it by the age of ten or so. Children don't have to make the choices we do... They already f-know it in advance." p. 653
Although linguists may be justified in being f-annoyed that little tykes know more about language than they do, Jackendoff's use of the term f-know is not an abbreviated expletive. The f actually stands for functional, and the point is that kids seems to have some functional knowledge of language structure (they f-know it) when they approach the task of language acquisition. This, of course, is not a new claim. I just like the way Jackendoff f-puts it. :-)
If you haven't read Ray's book, Foundations of Language, or at least the precis in BBS, it is worth a serious look. Lot's of ideas that make contact between linguistics, psycholinguistics, and neuroscience.
Précis of Foundations of language: brain, meaning, grammar, evolution.
Behav Brain Sci. 2003 Dec;26(6):651-65; discussion 666-707.