inexorabile = relentless; lit. that cannot be moved by entreaty. in (not) + ex (easily) + orabilis (entreated).
For oro cf. ōs = mouth; orator = speaker; oratio = speech.
fatum = fate, i.e. of death, as the common lot of all men, the decree of nature.
fatum = that which is said, espec. prophetically. √fa, φα. Cf. for (fā-ri), speak; fā-ma, report; fā-bula, a story; in-fans, that cannot speak; fā-cundus, eloquent.
strepitum = roar; lit. a wild, confused noise, din of any kind; cf. obstreperous.
Acherontis = Acheron = (a) a river in the Lower World; (b) the Lower World itself. Perh. Acheron = ὁ ἄχεα ῥέων = the stream of woe; cf. Κωκυτός = Cocytus, river of wailing.
(ii.) Translation.— You cannot be in doubt about the principal subject and predicate. Felix is the only word outside the subordinate clause from qui . . . avari. The sense, too, of these lines is clear, so you may translate at once; but you must take special care to use dignified and appropriate language:—
Happy the man who has availed to know the causes of things, and so trampled under foot all fears and fate’s relentless decree, and the roar of insatiate Acheron.
II. Fortunatus et ille, deos qui novit agrestes,
Panaque, Silvanumque senem, Nymphasque sorores!
(i.) Vocabulary.—