أَوْ يَكُونَ عَلَى أَقَلَّ مِنْ ثَلَاثَةِ أَحْرُفٍ، أَوْ تَضَمَّنَ مَعْنَى الْحَرْفِ.
Translation: Or its form has fewer than three letters, or it incorporates a meaning associated with a particle.
The text names further resemblances to particles. It does not mean every two-letter noun must be indeclinable; for example يَدٌ is declinable.
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The indeclinable noun: definition, rule and eight classes
أَوْ يَكُونَ عَلَى أَقَلَّ مِنْ ثَلَاثَةِ أَحْرُفٍ، أَوْ تَضَمَّنَ مَعْنَى الْحَرْفِ.
Translation: Or its form has fewer than three letters, or it incorporates a meaning associated with a particle.
The text names further resemblances to particles. It does not mean every two-letter noun must be indeclinable; for example يَدٌ is declinable.