Another linguistic oddity manifested by the Tel Dan fragment is its use of qdm as an adverb ("formerly" according to the translation of Naveh). It should be observed that an adverb qdm doesn't exist in Aramaic: such a function is fulfilled by the noun qdm in the plural, qdmn, qdmyn, or the singular but with the presence of a preposition. It isn't amiss to note that an adverb qdm is documented in biblical Hebrew, usually meaning "eastward", but with the meaning "long ago" in two psalms (74:2 and 119:152), both of which are post-exilic.Garbini is referring to the phrase ויעל מלך ישׁראל קדם בארק אבי, "the king of Israel came formerly into the land of my father." The gravamen of his argument is that the use of adverbial qdm here is a linguistic novelty in Aramaic, but is attested in later Biblical Hebrew; this suggests the hand of a forger.
Garbini evidently forgot the use of adverbial qdm in the Tell Fakhariye inscription, line 15: אל זי קדם הותר, "he made it (the statue) better than it was formerly." This attestation deflates this whole line of argument.
But even if there were no other attestation, the argument is weak. Are we to assume that no ancient text can evince a linguistic novelty? Are we to assume that all connections of ancient Aramaic with Biblical Hebrew are inauthentic? That would undermine quite a few Old Aramaic texts. The fact is that Garbini, once again, is caught reaching for insubstantial reasons to disqualify a text just because it is inconvenient to his own presuppositions.
It is worth asking, however, what possible linguistic phenomena should raise our suspicions. First, I would look for phonological or orthographic anomalies; Old Aramaic orthography has certain differences from later Aramaic. For instance, if the Tel Dan inscription had used the form ארע "land" instead of ארק, that would raise very serious questions. (But in fact there are no such anomalies.) Next, I would look for morphological anachronisms. If the Tel Dan inscription had used the suffix hwn "their," instead of hm, I would be suspicious. Also, I would look at the syntax. If the noun phrase מלך ישׂראל, "king of Israel" were written מלכא די ישׂראל, this might point to a forger, as would, for instance, ביתה זי דוד or the like instead of בת דוד, "house of David." There are no anomalies of this kind.
In my opinion, Garbini has failed to prove his case. Linguistically, the Tel Dan inscription gets a clean bill of health. This alone is not enough to prove authenticity. I also gave (and give) the James ossuary a clean bill of linguistic health, and there is now plenty of evidence to suggest that it is a forgery. But since the Tel Dan inscription, unlike the ossuary, was discovered in situ (and no one has brought evidence to deny that fact or to suggest that a conspiracy existed among the excavators), this in itself is enough to warrant the full use of the Tel Dan inscription in historical reconstruction.