After the causative of the verb kṛ , the ātmanepada is used, when it has the word mithyā 'incorrect', as an upapada or dependent word and is used in the sense of 'repeated wrong utterance', even when the fruit of action does not accrue no the agent.,
The phrase 'of the causative ending in णि' is to be supplied from sutra 1.3.67. The aphorism is restricted to the cases where the fruit of the action does not accrue to the agent. The word abhyasa means doing again, or repetition; as पदं मिथ्या कारयति means 'he repeatedly pronounces the word incorrectly, that is with wrong accent, &c., not once but constantly'.
Why do we say 'when the word mithya is used as an upapada?' The causative of kri will take parasmaipada when it has any other upapada. Thus पदं सुष्टुम् कारयति 'he pronounces the word correctly.'
Why do we say 'of the verb कृञ्? Because the causative of any other verb used along with the word mithya will not have atmanepada; as पदं मिथ्या वाचयति 'he repeatedly utters the word wrongly.' Here वच् takes parasmaipada.
Why do we say 'repeatedly'? For if the incorrect utterance is not habitual, then parasmaipada will be used; as पदं मिथ्याकारयति 'he pronounces wrongly, not always but once.',