Rule. A pluta vowel (three mātrās, written 3 after the vowel) and a pragṛhya vowel (a vowel given that name by SK 100–109) invariably (nityam) keep their own form when any vowel (ac) follows. Keeping one's own form is prakṛtibhāva: no sandhi of any kind takes place. The word prakṛtyā, "in its own form", is not written in this sūtra; it comes in by anuvṛtti from an earlier rule. Bhaṭṭoji says the two kinds of vowel "will be defined later": the pluta by SK 93–97, the pragṛhya by SK 100–109.
Example.
ehi kṛṣṇa3 atra → the final a of kṛṣṇa is pluta (SK 95, calling from afar) → a3 + a would take savarṇadīrgha → pluta blocks it → ehi kṛṣṇa3 atra.
harī + etau → the ī of the dual harī is pragṛhya (SK 100) → ī + e would take yaṇ → pragṛhya blocks it → harī etau.
Applies when / blocked by. Only when a vowel follows; before a consonant there is no vowel sandhi to block. Why nityam, "invariably"? Because SK 91 is a rival: it also keeps a pada-final ī in its own form, but shortens it as well. SK 91 is the later sūtra, so by paratva it would win, and harī etau would lose its long ī. The word nityam says that the bare prakṛtibhāva of this rule — no shortening — is the one that must apply. Bhaṭṭoji's commentators put it this way: only the bare prakṛtibhāva of SK 90 is fit to apply here, and nityam is what makes it apply.
Watch out. prakṛtibhāva is not a change; it is the absence of one. When you derive a form, you still write the step, because it records that a rule of chapter 3 was set aside and why.
The pluta and pragṛhya [vowels] will be defined [later]; they, when a vowel (ac) follows, invariably remain in their natural form (prakṛtyā). [Examples:] ehi kṛṣṇa³ atra gauś carati ("Come, Kṛṣṇa! Here a cow is grazing") — [the protracted a of kṛṣṇa³ does not combine with the following a]. harī etau ("these two, Hari…") — [the pragṛhya ī does not become yaṇ]. Why "invariably (nityam)"? So that in harī etau and the like, precisely this prakṛtibhāva [taught here] should obtain, and the prakṛtibhāva combined with shortening (hrasva) by iko 'savarṇe (SK 91) should not arise.
plutapragṛhyā aci nityam — As for "now [the section on] prakṛtibhāva": supply "is [now] examined (nirūpyate)." "The pluta and pragṛhya [vowels] will be defined": that is, [the pluta] by dūrād dhūte ca [A 8.2.84] and the like, and [the pragṛhya] by īd ūd ed [dvivacanaṃ pragṛhyam, A 1.1.11] and the like. "In their natural form (prakṛtyā)": because [the word prakṛtyā] recurs (anuvṛtti) from prakṛtyāntaḥpādam [A 6.1.115] — this is the idea. It means: they remain established in their own nature (svabhāva); that is to say, no sandhis take place.
"ehi kṛṣṇā³ atra": by dūrād dhūte ca, the a after the ṇ [of kṛṣṇa, in calling from afar] is pluta; for that [protracted a] there is no savarṇadīrgha before the [following] a. "harī etau": by īd ūd ed [dvivacanaṃ pragṛhyam], the ī after the repha [of the dual harī] is pragṛhya; for that [ī] there is no yaṇ substitution. — Objection: since in the previous sūtra, sarvatra vibhāṣā, the word vibhāṣā is not svarita [not an adhikāra], its discontinuance (nivṛtti) is already established; so what is the purpose of mentioning nityam here? — thus he asks, "why nityam?" He gives the answer with "harī etau": only when nityam is mentioned does, in harī etau and the like, this bare prakṛtibhāva [taught by] plutapragṛhyā aci alone obtain — this is the meaning. The word "yathā" [in Bhaṭṭoji's "yathā syāt"] is in the sense of fitness (yogyatā): this prakṛtibhāva alone is fit to obtain, and it would obtain [only] when nityam is mentioned — this is the meaning. So too further on in cases of this kind.
He shows what is excluded by [Bhaṭṭoji's] word "eva (alone)" with "ik": the prakṛtibhāva combined with shortening (hrasva), to be taught by iko 'savarṇe śākalyasya hrasvaś ca [A 6.1.127], should not arise (mā bhūt = should not occur). [In "mā bhūt,"] after the particle māṅ the aorist (luṅ) [is used]; [this is] an exception (apavāda) to [the use of] all the [other] tenses (lakāras). Were nityam not stated, then by posteriority (paratva) the prakṛtibhāva accompanied by the Śākala shortening would present itself [since iko 'savarṇe is the later rule]. But when nityam is stated, by the very force (sāmarthya) of that [word], the bare prakṛtibhāva [taught by] plutapragṛhyā overrides even the later Śākala [rule's] prakṛtibhāva combined with shortening — this is the meaning.