Although Augustine had a good mind yet he never fully mastered the the Greek language, and he later had reason to regret this fact.
Contrary to the custom in Rome of his day, the classical schooling that Augustine received in colonial North Africa was conducted principally in Latin rather than in Greek. Augustine later wrote in his Confessions that his first Greek teacher was a brutal man who constantly beat his students. As a result, Augustine rebelled by vowing never to learn Greek.
After he became a Manichaean, Augustine continued to read philosophy, but was soon hampered by his not knowing much Greek. When aged in his twenties he would have found it difficult to read a Greek philosophical or theological text, and when later as a bishop and an author he realised that he really needed to know Greek, by then it was impractical for him to undertake a formal study of it.
Although he gradually acquired some use of the language, he was never really at home in it. His acquaintance with Greek literature was very limited. It has been questioned, for example, whether he was able to use, in the original, either the Hebrew or Greek Scriptures. Apparently, he was in the habit of using translations of the Greek author, Plato (Confessions 8, 2)
On the other hand, however, Greek words frequently occur in his writings correctly rendered and discriminated. And in his Letter 59, he mentions to Marcellinus about his taking up the Psalms in Greek translation and finding, in reference to certain difficulties, that it agreed with the Vulgate (i.e., the Latin translation of the Bible by his colourful contemporary, Jerome).
One particular mistranslation of Greek by Augustine, however, concerned a significant theological insight by Saint Paul. Augustine incorrectly translated Romans 5:12 to say that the human race committed sin through Adam, and not merely because of Adam. Augustine took the verse to mean that every human being was spiritually present in Adam himself, and therefore directly responsible for the sin of Adam. In fact, the apostle Paul was merely saying that, as a result of the sin of Adam, death came into the world and we have all suffered as a result. Augustine's mistranslation had an unfortunate effect on his doctrine of original sin by making his writing on this subject harsher than it should have been. This has led some modern critics to reject this doctrine completely.
From references that Augustine made in his early writings, scholars generally hold that he was adequately instructed in Greek grammar but beyond that, however, his knowledge was insufficient for a thorough comprehension of Greek books, and especially of those written in the Hellenistic dialect. In later life he became much better at Greek, and was at least able to check Latin translations competently against their Greek originals. At this stage, Augustine listed in his Retractions some the mistakes that he made in his early works through his ignorance of Greek.
The after-effect of his having had in his childhood a teacher of Greek who was cruel to his students far outlasted the seventy-six years that Augustine spent on this earth. Because Augustine was such an influence on the thought and scholarship of the Church in the West during the Middle Ages, his dislike of Greek had lasting consequences. His low motivation towards the Greek language and his recourse to Greek texts less frequently than would otherwise have been the case unduly influenced subsequent Christian authors of the West. Because of Augustine, they disregarded the theological writings of the Greek-writing Church Fathers of the East more than would have happened otherwise. One un-named teacher of Greek in the life of the young Augustine has much for which to answer, but Augustine should also share some of the blame!
By way of an aside, it is interesting to note that Augustine was not an exception amongst the early Fathers of the Church for not knowing Hebrew. Of the Greek Fathers only Origen, and of the Latin Fathers only Jerome knew anything of that language. Augustine mentioned his lack of Hebrew more than once (Enarrationes in Psalm 86, 7 and De Doctrina Christiana 2, 22). He recommended to others a knowledge of Hebrew, as well as of Greek, so as to avoid his own frustration with "the endless diversity within Latin translations [of the same work]." (De Doctrina Christiana 2, 16).