Case markers from adpositions

[Ryan of Tinellb]

I'm given to understand that case markers are derived from adpositions. According to WALS, adpositions are evenly spread between pre- and post-positions, but case markers are overwhelmingly suffixes. Can adpositions be easily moved around, or am I basically forced to start with postpositions? Whence would, say, English derive a suffixed ablative case marker?

[Salmoneus]

I'm given to understand that case markers are derived from adpositions. According to WALS, adpositions are evenly spread between pre- and post-positions, but case markers are overwhelmingly suffixes. Can adpositions be easily moved around, or am I basically forced to start with postpositions? Whence would, say, English derive a suffixed ablative case marker?

AIUI, adpositions tend not to move around much per se. However, adverbs tend to move around, and adverbs can easily become adpositions, and I guess the opposite sometimes happens too. So in moving from prepositions to postpositions, you'll probably be replacing one set with another. That new set can come directly from unanalysable adverbs, or they can come from prepositional phrase, for example. They can also come from circumfixes that derive from nouns, adjectives, adverbs, etc, that lose the original prepositional element.

For instance, let's derive an English caritive. We'll start with the noun, 'side'. From this we form the prepositional phrase 'at side' - "he walked down the road, dog at side". This prepositional phrase naturally follows the noun (or the verb), so if we conjoin the preposition to the noun to form "aside", we have a postposition (or adverb, when following a verb). That postposition can then come to have a caritive sense: "I like dinner, potatoes aside" can come to mean "I like dinner without potatoes" very easily. That postposition can then become a suffix: "He returned the belongings watch aside" can become "he returned the belongings watchaside".

Or we could keep the same preposition by rerouting it through, say, a phrasal verb. So "out" is both a preposition ("I kick the ball out of the ground") and an adverb ("I kick the ball out"). In the latter form, it can become locked in a phrasal verb - "I run the time out". But now you have something very close to a postposition, because "I run the time out" is only a small semantic step from meaning "I run out of time". So "I ate the-suppliesout" (or "I ate the-suppliesup") can become a case for privation or departure.

[WeepingElf]

It seems as if prepositions and postpositions are roughly equally frequent, but postpositions tend to agglutinate to the noun giving case suffixes, while prepositions don't, and just continue being prepositions rather than becoming case prefixes, which seem to be vanishingly rare.