91 in Elizabethan English, but not in present-day English. Using Chomsky’s strength metaphor, we can say that the Tns affix carried by a finite T was strong in Elizabethan English, but is weak in present day English. Because the affix was strong in finite clauses in Elizabethan English, it could attract a verb to move from V to T; but because the affix is weak in present-day English, T can only be filled by an auxiliary which is directly merged in T, not by a verb moving from V to T. More generally, we can suppose that there is parametric.
