"Twere..."See in text(A Valediction: Forbidding Mourning)
This is a contraction of “it were.” Donne’s need to shorten these two already short words comes from the meter, or rhythm, of the line which is written in iambic tetrameter, meaning there are four stressed syllables per line. “‘Twere” fits better because the stresses in this poem fall on the second syllable of the line. Since the “pro” of profanation must be stressed, “it were” would not work metrically because then both “were” and “pro” would be stressed throwing off the meter of the line.
