Regular Sound Change
Sound change is regular: within the same linguistic conditions, a change affects a sound systematically across words rather than mutating each word independently.
If Sanskrit *hasta* changes according to a particular sound pattern, *aṣṭa* cannot simply opt out: under the corresponding conditions, it must change too. What looks like the separate history of two words is constrained by one shared rule.
E1The sound changes, not the word
The unit of change is a sound in a defined environment—not an isolated vocabulary item. Once a sound shifts under certain conditions, every word containing that sound under those same conditions should reflect the shift. This regularity turns apparently scattered mutations into a testable system: one proposed rule must account for multiple words at once.
E1Where it shows up
*Hasta* and *aṣṭa* rise together
The paired Sanskrit forms provide the crucial check. A pattern proposed from *hasta* predicts the corresponding treatment of *aṣṭa*; the second word is evidence for or against the rule inferred from the first.
E1Regular does not mean unconditional
The claim holds only when the sound and its conditions are genuinely the same. A difference in linguistic environment can produce a different outcome, so surface resemblance alone does not establish that two words must share a history.
E1Turn one observation into a prediction
When you think you have found a sound change, write it as a conditional rule, then search for another word containing the same sound under the same conditions. Treat that second word as a prediction test, not as a new exception to explain separately.
E1Episodes that teach this
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Proto-Indo-European: The Mother of All Languages | Future IQ
· explained at 9:50
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if there is a change that is happening like this right it applies across the language right