My most recent target has been close. Of course, there is the verb as in close the door, and the noun meaning `the end' (e.g., the close of the day); these are clear cut. But then there are examples like these:
- Closer to the end, I'll come and get you.
- The bank is close to the store.
- I walk to my university because it is very close by.
- Put it close to the door.
- She followed very close behind the taxi.
Adjectives, you see, are supposed to be predicative. In 1, for example, something should be closer to the end, in the same way that in intrigued, she examined the walnut desk, she is curious. You can't have *intrigued, the walnut desk was examined, because the syntax suggests that the walnut desk is intrigued by something and out experience with desks belies such a notion.
Adjectives simply don't work in this non-predicative adjunct function, but prepositions do. Semantically speaking, prepositions also tend to be locative, and close is certainly that. On the other hand, prepositions, aren't supposed to be gradable or modified by very, and that makes close a highly unusual preposition (near & far are in the same boat).
Adding to the confusion, Rodney Huddleston suggests to me that close by, despite having a space is actually a compound like nearby.
And then there's 5. Here, close appears to be functioning as a modifier for the preposition behind. There are a small number of prepositions that do function as modifiers in prepositional phrases (e.g., over in the corner), but perhaps it is better to consider close an adverb here since it's similar to closely, right, immediately, just, all of which are adverbs. As Huddleston also points out to me, you couldn't substitute near there.
I've tried to capture all this in the Simple English Wiktionary entry for close.
Jespersen, O. (1924). The Philosophy of Grammar. London: Allen and Unwin.
Jackendoff, R.S. (1973). The base rules for prepositional phrases. In: Stephen R. Anderson and Paul Kiparsky (eds.), Festschrift for Morris Halle. New York: Holt, Rinehart and Winston, 345-356.