If one regards a language as vocabulary cast into the mould of a particular syntax (with functional items maintaining the basic structure of a sentence and with the lexical items filling in the blanks), syntactic change no doubt plays the greatest role in modifying the physiognomy of a particular language. Syntactic change affects grammar in its morphological and syntactic aspects and is one of the types of change observed in language change.
If, however, one pays close attention to evolutions in the realms of phonology and morphology, it becomes evident that syntactic change can also be the result, rather than the cause, of profound shifts in the shape of a language, i.e. the effect of phonological change can trigger morphological reanalysis which can then engender changes in syntactic structures.
Syntactic change is a phenomenon creating a shift in language patterns over time, subject to cyclic drift. The morphological idiosyncrasies of today are seen as the outcome of yesterday's regular syntax. For instance, in English, the past tense of the verb to go is not goed or any other form based on the base go, as could be expected, but went, a borrowing from the past tense of the verb to wend.
Over time, syntactic change is the greatest modifier of a particular language. Massive changes may occur both in syntax and vocabulary and are attributable to either creolization or relexification. Some theories of language change hypothesize that it occurs because the grammatical input children receive is ambiguous, and so they analyze the underlying grammatical construction in a different manner than previous generations did. This reanalyzed grammar, in turn, may create new ambiguities, which subsequent generations may analyze in yet another manner.
In some cases, change can happen in a cyclic manner. For example, prepositions can become reduced over time, until they are reanalyzed as case markers affixed onto the adjacent nouns. These case markers, in turn, may be lost over time, which will lead to the introduction of new prepositions.
An example of syntactic change in English can be seen in the development from the verb second (V2) word order, used before the 15th Century, to the modern word order. Just as with other Germanic languages, Old and Middle English had V2 word order. An example from Middle English is shown in (1), where nu 'now' is in first position, and the verb loke 'look' is in second position.
(1) Nu loke euerich man toward himsuelen.
now look every man to himself
'Now it's for every man to look to himself.' (as cited in Roberts 2007: 59)
Even though V2 was lost, verb raising was maintained in the 1600s in Early Modern English. Unlike in Modern English, the verb preceded adverbs and negation, as shown in (2). This word order is still apparent in Shakespeare's writing.
(2) if I gave not this accompt to you
'if I didn't give this account to you' (1557: J. Cheke, Letter to Hoby; Görlach 1991: 223, as cited in Roberts 2007: 57)