णौ चङ्युपधाया ह्रस्वः

A short is substituted for the vowel, standing in a penultimate position, the Causative अङ्ग (stem), when the affix चङ् (sign of the Reduplicate Aorist of the Causatives) follows.
As अचीकरत्, अजीहरत्, अलीलवत्, अपीपठत् ॥ Here the rules of reduplication and shortening of the penultimate both present themselves simultaneously. The rule about shortening, being subsequent in order, is applied first, and then the reduplication takes place. Thus कारि + अत् = कार् + अत् (णि being elided 6.4.51) = कर् + अत् (shortening 7.4.1) = चकर् + अत् 6.1.11 = चिकर् + अत् 7.4.93, 7.4.79 = चीकरत् 7.4.94. The necessity of maintaining this order will appear from the following considerations. The Causative stem of अट् is आटि; the Aorist of which is आटि + चङ् + त् = आट् + अत् 6.4.51. Now if reduplication took place first we shall have आटिट् + अत् 1.1.59 the elided इ will be present for the purposes of reduplication) and as the penultimate is short already, the form will be आटिटत्, which with the augment अ will be अ + आटिटत् = आटिटत् ॥ This is a correct form, so far as it goes; but when the augment is elided in connection with the negative मा, the form will be मा भवान् आटिटत्, the correct form however is मा भवान् अटिटत् with a short अ, which can be formed if we shorten first and then reduplicate, as आट् + अत् = अट् + अत् = अटिट् + अत् = अटिटत्, which with the augment अ, will be आटिटत् ॥ In fact, though the reduplication of the second syllable 6.1.2 is a nitya rule, because it applies even where a penultimate is shortened and where not, yet the rule about shortening takes effect first, because the author has himself indicated this, by making the root ओण have an indicatory ऋ in the Dhatupatha (See 7.4.2 about ऋदित् verbs). For had the reduplication taken place first in the case of ओण् (Bhuadi. 482), as ओणिण्, there would be no long vowel to be shortened, and there would be no necessity of the probibition 7.4.2.

Objection: Why do we say 'in the Causative'? The चङ् Aorist is formed of Causatives only (See 3.1.48), the only exceptions being the simple roots श्रि, ब्रु, सु, धेट् and श्वि, none of which have any vowel in the penultimate and कम् and सुप् have already short upadha. The sutra चङ्युपधाया ह्रस्वः would have been enough. Answer: Had the sutra been, as proposed, then it would have meant, 'that which is penultimate when चङ् follows, should be shortened'. Therefore in the Aorist of the Causative of लू, we have लू + णि + चङ् + त् = लू + इ + अत् here the penultimate with regard to चङ् is ऊ, which would be shortened; debarring Vriddhi and आव् substitute, and there would have come the उवङ् substitute instead. But that is not the case. We have अलीलवत्, and not अलुलुवत् ॥ Similarly, in the Aorist of the Causative of दा, we shall have दा + इ + अत्, and आ being shortened we have द + इ + अत, so that we cannot add the augment पुक् 7.3.36, and form अदीदपत् ॥ And the forms like अपीपचत् (पच् + इ + अत्) would not at all admit of shortening.

Why do we say when चङ् follows ? Observe कारयति, हारयति where there is no shortening in the Present Tense. Why do we say of the penultimate? Observe अचकाङ्क्षत् from काङ्क्ष्, and अववाञ्छत् from वाञ्छ, where the penultimate being a consonant, there can be no shortening. Had the word penultimate been not used, the vowels of these would be shortened. And the word 'upadha' is also absolutely necessary for the sake of the subsequent sutras like 7.4.4, and it prevents shortening in the above case, which would have otherwise taken place, on the maxim येन नाव्यवधानं तेन व्यवहितेऽपि वचनप्रामाण्यात् which qualifies 1.1.67.

Vart:- The rule of shortening of the penultimate applies to the चङ् Aorist of the Causative of the Causative i. e. to the double Causative even. Thus अवीवदत् (= वादितवन्तं प्रयोजितवान्) वीणां परिवादकेन ॥ Otherwise वादि + इ + अत् = वाद् + इ + अत् = वाद् + ० + अत् ॥ Here the elided इ being sthanivat, will prevent shortening, or because the stem has lost a simple-vowel (अग्लोपिन्), it will not be shortened. The present vartika makes it so however.