How are ancient scripts deciphered?
By finding a foothold — a name, a known language, or a structural pattern — and then working outward under constant constraint, since every proposed reading must remain consistent with every other text.
The three things a decipherer needs, and rarely has all of:
Enough text. A handful of short inscriptions may be undecipherable in principle, because there is not enough material to test hypotheses against.
A known language behind the script. Egyptian hieroglyphs fell because Coptic preserved a late form of the language; Linear B fell when the language turned out to be an early form of Greek. A script recording an unknown language is far harder, which is why Etruscan can be read aloud and not understood.
A bilingual text or a known name. The Rosetta Stone provided the same decree in Greek and in two Egyptian scripts, and Champollion's breakthrough came from recognising royal names in cartouches — proper nouns are the usual wedge, because they must sound approximately the same in any script.
What decipherers actually do: count signs, since an alphabet has a few dozen, a syllabary perhaps 40 to 90, and a logographic system hundreds; look for grammatical patterns, endings that vary while stems stay constant; identify numbers, which have distinctive repetitive forms; and use the context of the object — a tax record, a dedication, a king list — to guess content.
The case worth knowing. Linear B was cracked by Michael Ventris, an architect, using a grid method developed partly by Alice Kober, who had systematically identified sign patterns indicating inflection before her early death. Ventris expected the language to be Etruscan; the grid told him it was Greek, and he accepted the evidence against his own hypothesis.
What remains undeciphered: Linear A, the Indus Valley script, Rongorongo and several others — held up variously by short texts, unknown languages, or uncertainty about whether the marks encode language at all.
Machine learning is now being applied, and so far assists rather than replaces the structural reasoning.