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2.3.14 क्रियार्थोपपदस्य च कर्मणि स्थानिनः

The un-implied verb with क्रियार्था उपपद has चतुर्थी. The unimplied sense means that the literal meaning of the verb is not implied (it's secondary). For example, in फलेभ्यो याति, गमन(याति) is unimplied with क्रियार्था उपपद, hence चतुर्थी but in फलानि आहुर्तं याति (with implied आहुर्तं) has द्वितीया.