William Shakespeare and his contemporaries of the late sixteenth and early seventeenth century wrote sentences like these:
(1) a. for the eye sees not itself / But by reflection (Julius Caesar I.ii.52)
b. What means this shouting? I do fear, the people Choose Caesar for their king. (Julius Caesar I.ii.79–80)
We can still understand such sentences, but these two examples illustrate three instances of syntax that is no longer normal in Present-day English. To start with, sentence negation is no longer expressed by placing the negator not after a main verb. Today, we can only place a negator after an auxiliary verb, as in the eye cannot see itself, a sentence which Shakespeare could also have written. Similarly, we need an auxiliary to form the question of (1b), turning it into What does this shouting mean? And rather surprisingly, given that he could leave out auxiliaries where modern-day speakers need one, Shakespeare could use an auxiliary where we could not – I do fear could indeed be uttered today, but only in emphatic speech, as in I do not fear that Brutus will betray Caesar, but I DO fear that the people will choose Caesar for their king.
Although we cannot assume that people in Tudor England produced speeches exactly like Shakespeare's in their everyday language, we can assume that these particular constructions reflected syntactic possibilities in more pedestrian spoken language. The essence of the generative approach to analyzing linguistic change between period A and period B is to make a formal and explicit description of the differences between the internalized grammar of a speaker of stage A and that of a speaker of stage B.
Generative theory is primarily focused on synchronic analysis, and shifts in the treatment of changes such as those illustrated by the sentences of (1) reflect developments in the synchronic theory. However, diachronic generative linguists assume that the results of their studies can help shape synchronic theory as well as elucidate the history of a given language and add more generally to our understanding of how languages change. As we'll see below, it has been argued that a single change in a hypothesized parameter setting between Shakespeare's period and later led to a cluster of apparently unrelated changes, including the loss of a speaker's ability to produce questions and negatives without an auxiliary.