What then is the law? (τ ουν ο νομοσ?). Or, why then the law? A pertinent question if the Abrahamic promise antedates it and holds on afterwards.

It was added because of transgressions (των παραβασεων χαριν προσετεθη). First aorist passive of προστιθημ, old verb to add to. It is only in apparent contradiction to verses Galatians 3:15, because in Paul's mind the law is no part of the covenant, but a thing apart "in no way modifying its provisions" (Burton). Χαριν is the adverbial accusative of χαρις which was used as a preposition with the genitive as early as Homer, in favour of, for the sake of. Except in 1 John 3:12 it is post-positive in the N.T. as in ancient Greek. It may be causal (Luke 7:47; 1 John 3:12) or telic (Titus 1:5; Titus 1:11; Judges 1:16). It is probably also telic here, not in order to create transgressions, but rather "to make transgressions palpable" (Ellicott), "thereby pronouncing them to be from that time forward transgressions of the law" (Rendall). Παραβασις, from παραβαινω, is in this sense a late word (Plutarch on), originally a slight deviation, then a wilful disregarding of known regulations or prohibitions as in Romans 2:23.

Till the seed should come (αχρις αν ελθη το σπερμα). Future time with αχρις αν and aorist subjunctive (usual construction). Christ he means by το σπερμα as in verse Galatians 3:16.

The promise hath been made (επηγγελτα). Probably impersonal perfect passive rather than middle of επαγγελλομα as in II Macc. 4:27.

Ordained through angels (διαταγεις δι' αγγελων). Second aorist passive participle of διατασσω (see on Matthew 11:1). About angels and the giving of the law see on Deuteronomy 33:2 (LXX); Acts 7:38; Acts 7:52; Hebrews 2:2; Josephus (Ant. XV. 5. 3).

By the hand of a mediator (εν χειρ μεσιτου). Εν χειρ is a manifest Aramaism or Hebraism and only here in the N.T. It is common in the LXX. Μεσιτης, from μεσος is middle or midst, is a late word (Polybius, Diodorus, Philo, Josephus) and common in the papyri in legal transactions for arbiter, surety, etc. Here of Moses, but also of Christ (1 Timothy 2:5; Hebrews 8:6; Hebrews 9:15; Hebrews 12:24).