As mentioned previously, Word Order cannot fully account for the allocations of arguments to different ‘slots’ in a predicate. For example, some languages enjoy basically free word order. While the unmarked order in Latin is SOV, OSV is also grammatical.
(3) svennus catulum calcitravit
(3b) catulum svennus calcitravit
How then do we know that (3b) does not describe the puppy visiting his revenge upon Sven?
That sentence would be written as follows:
(4) catulus svennum calcitravit
(4b) svennum catulus calcitravit
Notice the difference? The object takes -um as an ending, whereas the subject takes -us. In this way, the language can move words around in a sentence without affecting the literal meaning. This is not to say that movement is pointless; it can do all sorts of things in pragmatics and aesthetics (see The Aeneid for plenty of that). The more specific roles a case system distinguishes, the more movement is possible. English, for example, has only two cases, and only on pronouns at that: Nominative and Accusative(and a possible dative case). As such, movement is relatively restricted. As a general rule, movement of an argument is possible if and only if there are no other non-conjoined sentence arguments which take the same case as that argument, and thus must have their function determined by order.
In English, for example, objects can be moved in ditransitive(two-argument) constructions if and only if case is marked in same way.
(5) I gave Jeff the money
(5b) I gave the money to Jeff
(6) Jeff, I gave the money – The fronted component takes the same case as another possible case (the vocative), and thus one cannot tell whether (6) was created via movement from (5) or from
(7) I gave the money, Jeff – creating an ambiguity as to whether I am telling Jeff that I gave the money, or stating that I gave the money to Jeff
(7b) to Jeff, I gave the money – 7b, however, is unambiguous because Jeff is marked by ‘to’.
Therefore, (7b) is permissible as a permutation of (6b), but (7) is not a permissible permutation of (6)
In The Old Tongue, Case Marking allows for almost unlimited movement, due to a robust marking system. In fact, there are 20 cases marked for, and putting them here will be unwieldy. But you can find the case system here.