How did St. Jerome defend the Blessed Virgin's virginity post partum in light of Matt. 1:25, which says St. Joseph "knew her not till she brought forth her firstborn son"?
St. Jerome proved that Matt. 1:25 neither affirms nor denies her virginity post partum.
From the phrase ‘before [έως] they came together’ it does not follow that they came together afterwards; Holy Scripture merely intimates what did not happen [up until the time of Jesus's birth].
Quod autem dicitur antequam convenirent, non sequitur quod postea convenerint, sed Scriptura, quod factum non sit, ostendit.
St. Jerome refutes this possibility by an example (ibid.):
If I say: ‘Helvidius died before he did penance for his sins,’ does it [necessarily/logically/always] follow that he did penance after his death?
and by citing other scriptural verses where doing (or not doing) something until a later time doesn't necessarily imply that one stops (or starts) doing it after said time:
Ps. 109:1: “Sit thou at my right hand, until I make thy enemies thy footstool,” and Gen. 8:6 sq.: “… the raven … did not return till the waters were dried up upon the earth.” Does it follow, he asks, that Christ will no longer sit at the right hand of God the Father when His enemies lie defeated at His feet? Or did the raven return to the ark after the waters were dried up?
—Pohle-Preus, Mariology, pt. 2 ch. 1 §3 "Mary's Perpetual Virginity", "Thesis III: The Blessed Mary remained a virgin after the birth of her Divine Son"