A recent post by Mississippi Fred MacDowell correctly notes that Jerome knew Hebrew well. This is true, and we know this for certain, but not for the reason he suggests: that there were not enough aids available at the time for Jerome to pass off the Vulgate as a translation from the Hebrew even if it wasn’t. Such aids were available, and Jerome used them to the hilt. But it remains true: he knew Hebrew very well. How do we know this?
The principal aids Jerome had at his disposal were two: “native informants,” that is, Jewish teachers; and "essentially literal" to "extremely literal" Jewish translations of biblical literature into Greek: Theodotion, Symmachus, and Aquila. It has been shown that Jerome relied especially on the latter two.
Jerome’s commitment to mastery of the Hebrew language began when he moved to the desert of Chalcis east of Antioch in 374 (he was 27 years old), continued when he returned to Rome (382), and did not stop when he settled in Bethlehem in 386 [corrected from 384: see Kevin's comment below]. He states his modus operandi clearly: sicubi dubitas, Hebraeos interroga (Epistulae 112:20) ‘when in doubt, ask the Jews.’ He mentions one of his teachers by name (a certain Bar-Hanina), and others by title only: a deuterotes = תנא (not in the technical sense of a scholar mentioned in a mishna or baraita, but in the sense of someone who was a transmitter of oral tradition, including exegetical tradition contained in the Midrashim); a scriba = ספרא (a teacher, not necessarily a rabbi), and a sapiens = חכים (a rabbi).
A careful reading of Jerome’s commentaries reveals that he became fluent in Hebrew. In theory, he might have translated not so much from the Hebrew as from Greek translations like that of Symmachus and Aquila. I will give an example of his reliance on Aquila in a future post (the famous case of Exodus 34:29). But statements in his commentaries show that Jerome, who said, ego philosophus, rhetor, grammaticus, dialecticus, hebraeus, graecus, latinus, trilinguis, really was. Jerome’s fluency in Hebrew allowed him to make the best use possible of the aids mentioned above. It goes without saying that Jerome’s understanding of Hebrew and of language in general was defective in the same way that other scholars’ understanding of the time was.
Kedar’s key paragraphs read as follows (all references are to Vallarsi’s edition of Jerome’s opera omnia, a pain in the neck since it means that references cannot be easily checked against the edition [PL] of Jerome’s works online):
Jerome’s seriousness and philological precision in reading the Hebrew text finds its expression when he is ready to forgo a christological interpretation of a verse because one silent letter is in the way: ‘(The Lord) declares to man what is his thought’ (Am 4:13) had been rendered by the LXX (in Jerome’s Latin translation): annuntians in homines Christum suum ‘. . . announcing his Christ to men’. Had the Hebrew letters been mšyḥ, he expounds, it could have stood for mašîaḥ ‘Messiah’; since, however, the sequence had a superfluous Hē’ as a second letter, i.e. mh šyḥ (HE secundum literam plus habens; 6, 278) Jerome feels obliged to correct this rendering (Vg adnuntians homini eloquium suum). Though having before him an unvocalized text, he frequently comments on the proper vocalization: MEM, JOD, MEM – si legatur MAIM ‘aquas’ significat, si MEJAM ‘de mari’, ‘if the sequence m-y-m is read mayim, it means ‘water’, if miyyām ‘from the sea’ (6, 131).
The fact that Jerome can compare three languages and the problems he faces as a translator, make him a keen observer of linguistic phenomena. He raises fine points of grammar and semantics. He notices the lack of the definite article in Latin (7,503) and comments on number and gender of nouns. Hebrew RUA ‘spirit’ is feminine; however, nobody should find this, in view of the Holy Spirit, scandalous: in Latin the word is masculine, in Greek neuter, so that we learn that deity has no sex (in divinitate enim nullus est sexus; 4, 485-486). Jerome is capable of making nice semantic distinctions between Hebrew synonyms (though he does not always pay attention to them in his translation): nĕbēlâ is the corpse of a beast which has died without shedding blood, ṭĕrēpâ is a corpse torn by beasts (5, 45). The verb znh ‘to fornicate’ denotes the conduct of a woman who lies with many men, n’p ‘to commit adultery’, a woman that deserts her husband for someone else (6, 140. ‛āwôn ‘iniquity’, antedates the laws, ḥeṭ’ “sin’, occurs after the law (6, 89). Jerome remarks on the polysemy of words: rûaḥ, according to context, may mean either spiritus ‘spirit’, or anima ‘soul’, or ventus ‘wind’ (5, 7). reḥem denotes vulva ‘womb’ and misericordia ‘mercy’ (6, 235).
There are hundreds of examples of the above kind. Jerome inadvertently demonstrates that he is able to find easily a Hebrew equivalent for a given Greek word whenever, in correcting the wording found in the lxx, he states what the Hebrew vocable, not found in mt, should have been, were the lxx correct (e.g. 4, 470). And thus we may believe him, that when he translated the Aramaic text of Tobit, he asked a Jew to give an improvised Hebrew rendering of it which Jerome, in his turn, dictated on the spot in Latin to his scribe (Prologus Tobiae). If we accept this evidence, it proves a high degree of proficiency.
 Benjamin Kedar, “The Latin Translations,” in Mikra: Text, Translation, Reading and Interpretation of the Hebrew Bible in Ancient Judaism and Early Christianity (ed. Martin Jan Mulder; CRINT 1/2; Assen/Maastricht: Van Gorcum; Minneapolis: Fortress, 1990) 299-338; 313-34.
 James Barr, “St. Jerome’s Appreciation of Hebrew,” BJRL 49 (1966-67) 281-302; “St. Jerome and the Sounds of Hebrew,” JSS 12 (1967) 1-36.
 Kedar, op. cit., 316-17.