«رُوَيْدَ» and «هَيْهَاتَ» retain fixed forms.
The noun associated with them receives its own case according to syntactic role: «زَيْدًا» is accusative; «زَيْدٌ» is nominative.
Do not confuse the verb-noun's fixed ending with the governed noun's case.
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An Indeclinable Word and a Declinable Noun
Asmāʾ al-Afʿāl — Definition, Uses and Parsing
«رُوَيْدَ» and «هَيْهَاتَ» retain fixed forms.
The noun associated with them receives its own case according to syntactic role: «زَيْدًا» is accusative; «زَيْدٌ» is nominative.
Do not confuse the verb-noun's fixed ending with the governed noun's case.