هَذِهِ يَدٌ. — This is a hand: yadu(n) is a nominative predicate.
رَأَيْتُ يَدًا. — I saw a hand: yada(n) is an accusative object.
أَمْسَكْتُ بِيَدٍ. — I held a hand: yadi(n) is genitive after bi-.
Thus letter count alone is not a reliable universal test. The book speaks of specific patterns of resemblance.
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The indeclinable noun: definition, rule and eight classes
هَذِهِ يَدٌ. — This is a hand: yadu(n) is a nominative predicate.
رَأَيْتُ يَدًا. — I saw a hand: yada(n) is an accusative object.
أَمْسَكْتُ بِيَدٍ. — I held a hand: yadi(n) is genitive after bi-.
Thus letter count alone is not a reliable universal test. The book speaks of specific patterns of resemblance.