In 1806, when William Wordsworth composed his sonnet, "Nuns fret not at their convent's narrow room," it was perfectly acceptable to say "nuns fret not," but not anymore. Now we would be obliged to say, "Nuns do not fret at their convent's narrow room." This is because, in order to transform a positive, declarative sentence into a negative, interrogative, or emphatic form, we need to insert a "do" when there is no other auxiliary to signal the tense.
The linguistic process at work is called "analogy." We want rules to apply across the board, and when they don't, we often try to change them. Most verb forms have auxiliaries. For example, we can easily transform "Nuns are fretting" to "Nuns are not fretting" without introducing a "stand-in." There are only two verb forms that have no other auxiliary and therefore need "do," the simple present and the simple past, as in "fret" or "fretted." Because we wanted these forms to match up with the others, we began to use "do" as a substitute for the "missing" auxiliary.
There is only one modern way of saying "Nuns fret not."
