The text explicitly distinguishes jāʾiz (permitted) from wājib (required) alteration. These are not interchangeable.
Where a change is optional, a form retaining hamzah can remain legitimate alongside a reduced pronunciation.
Where a rule makes substitution obligatory, the specified structural setting requires the altered form within the taught paradigm.
The general rule for a quiescent hamzah is described as permitted; the rule for a particular sequence of two hamzahs in one word is described as obligatory. Misreading that distinction distorts conjugation.
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Optional Versus Obligatory Changes
Rules of Hamzated Forms: Lightening, Substitution, Elision and Short Paradigms
The text explicitly distinguishes jāʾiz (permitted) from wājib (required) alteration. These are not interchangeable.
Where a change is optional, a form retaining hamzah can remain legitimate alongside a reduced pronunciation.
Where a rule makes substitution obligatory, the specified structural setting requires the altered form within the taught paradigm.
The general rule for a quiescent hamzah is described as permitted; the rule for a particular sequence of two hamzahs in one word is described as obligatory. Misreading that distinction distorts conjugation.