So my friend gaetanus and I were in a bar the other night, drinking beer, discussing Germanic and Semitic linguistic patterns, and studying German. We're reading through parts of a German translation of The Lord of the Rings to help us brush up since we both know the English text almost by heart. And I got to thinking about those separable-prefixed verbs German has. My thoughts are far from being fully worked out or refined; I'm just posting where they are at the moment.
Here's the situation: German has verbs with separable prefixes, like ausgehen, 'to go out'. 'I go out' is, simply enough, ich gehe aus. In the presence of other modifiers, the prefix is sent to the end of the sentence, hence Ich gehe morgen abend mit Elsa aus, 'I'm going out with Elsa tomorrow evening.'
I've always been fascinated by these verbs in German, and by what I think is a related issue of the distinction between adverbs and prepositions. In a way, prepositions are like adverbs that have a particularly strong association with a noun (its object). Hinter [behind] is a preposition when it has an object, and an adverb when it lacks one. I mean, why make new words for basically the same thing? 'You've fallen behind' (adverb) vs. 'You've fallen behind me' (preposition).
In the English sentence, I'm going out, 'out' is clearly an adverb. In the German sentence, Ich gehe aus, the word aus is part of the verb, but when separate it plays what boils down to the same role: it modifies the verb.
Ok, now watch what happens when I translate this sentence into German: 'The landlord delivers to Frodo the letter.'--Der Wirt stellt dem Frodo den Brief zu. You want to say that zustellen means 'to deliver [something] to [someone]', but be careful. The zu in zustellen does not correspond to the to in deliver to. In English the to is a preposition, and is grouped grammatically with its object; only as a whole does the prepositional phrase 'to most hobbits' modify 'applies' adverbially. Zu, on the other hand, modifies the verb directly, and the Hobbits are left to be an indirect object of the verb, rather than an object of a preposition.
So musing that the separable prefix is essentially an adverb that is now permanently associated with the verb, I went further and thought: Could the prefixes of some of these verbs have originally (i.e., in a pre-modern stage of the language) begun their lives as prepositions?
In Old English, there are two ways to construct prepositional phrases. Sometimes you find the preposition before its object as in English ("I walk along the beach"), but other times you will find the preposition after its object (as if to say "I walk the beach along"). What's that? Oh, yes, good catch: it is, of course, more properly a postposition. Either way, pre or post, works once you know what's going on. In fact, as gaetanus pointed out, the postposition construction has the convenient added feature of boxing in the modified text, so it is clear that it doesn't modify anything outside the box. What's more, the box is framed by verb + postposition, two elements working together from afar to do the same job: like parents at church sitting at opposite ends of a pew, with all their kids between them so they can keep them in line.
See if this works with our letter-delivery example. (Hopefully we'll have that letter fully delivered by the end of this post, so Frodo can get on with his story, and we with our own.) If we were to use a postposition instead of a preposition, the sentence would look like this (the German sentence follows again, to show the similarity):
The landlord delivers Frodo the letter to.
Der Wirt stellt dem Frodo den Brief zu.
So the successive stages of how words like to/zu were conceived may have gone thus: 1) 'To' is an adverb (thus modifying the verb) indicating direction of delivery, and is essential to the meaning of the verb (it cannot be omitted). 2) When the sentence contains more information determining the verb (like objects), the essential adverb may go before or after it (probably convention will settle on one of these options). 3a) The essential adverb begins to be associated, in the minds of speakers, less with the verb and more with its object. They form one unit, a 'prepositional phrase' which as a whole, modifies the verb. OR: 3b) The essential adverb is associated with the verb so closely it is now considered 'part' of it: a multiple word predicate, just like 'will be going'.
So, if any of this is plausible, it seems English followed the path to 3a, while German took the road to 3b, but really they both started out doing the same thing. I'm not an expert, of course, and this could all be wrong as likely as not, but it's something to look into.