4 Singing before the eternal majesty,
before > in front of
5 In their trinal triplicities on high;
trinal > triple triplicities > trios, triads (referring to the nine angelic orders of St Thomas Aquinas and others. The three Hierarchies are each divided into three Orders of Choirs: Seraphim, Cherubim, Thrones; Dominations, Virtues, Powers; Principalities, Archangels, Angels)
6 Yet wist no creature whence that heavenly sweet
wist > knew sweet > sweetness
7 Proceeded, yet each one felt secretly 8 Himself thereby reft of his senses meet,
reft > deprived, robbed senses meet > proper senses
9 And ravished with rare impression in his spirit.
112.40