The “inceptive” verbs were intransitive verbs formed from an adjective or verb with the sense of “begin to, grow, become”, as in: “begin to fall”, “grow pale”, “become light, lighten”. Inceptives can be broken up into two groups: verbal inceptives and adjectival inceptives. In the 1950s Common Eldarin: Verb Structure (EVS2) Tolkien gave two different ways to form inceptives: verbal inceptives formed with suffixes lu, ru, nu (PE22/136) and adjectival inceptives formed in ancient times with the consonantal suffixal -s (PE22/135).

Verbal inceptives are formed from verb roots, and indicate the beginning of an action: “stand” vs. “begin to stand, stand up”; “sit” vs. “begin to sit, sit down”. In essence, they turn a normally continuous action into an instant action marking the beginning of that activity. The common verbal inceptive suffixes are lu, ru, nu, often in combination with the prefix et- “out”:

The suffixes lu, ru, nu, mainly used after bases ending in sonants, and especially after final j of KAL-bases, had always an inceptive force: as in thillu, thilnu “shine out, appear (of stars etc.)”; √KOJ: et-koiru/lu “come to life”; √KUJ: et-kuinu “awake, wake up”; √KAJ: kainu “lie down”; √TUJ: tuilu “open (of buds, flowers)”. Contrast: koitā́ “bring to life”, kuitā́ “waken, rouse”; and káit-ă “lie, be on the ground” (PE22/136).

Sometimes ordinary u-verbs had a verbal inceptive sense, but this was not universal:

In many cases the u-verbs have an inceptive sense, as in: sedu, “go to rest”, kelu, “well forth, begin to flow”, tolu, “stand up”, khamu, “sit down”; a similar sense is often seen in the verbs showing affixes lu, ru, nu: as koiru “come to life”. It is probable again that the u is not the bearer of this significance which belongs to the consonant (in lu etc.), but that the frequent use in Eldarin of inceptive lu, etc. tended to colour the senses attached to u-verbs (PE22/135).

Adjectival inceptives were formed from adjectives, and indicate a transition to a state described by that adjective. It is functionally similar to the suffix “-en” in English: “white” → “whiten”, “red” → “redden” (though the English suffix is not very productive). These inceptives may be contrasted with causative verbs: “make white” (causative) vs. “become white” (inceptive). In the 1950s document EVS2, Tolkien said:

To this class [formative verbs] belonged also the inceptives formed with s. This s was added direct to the stem of adjectives, but in the case of verbal stems required an “extension”. Thus tañkās- “become firm, settle down”; niñkwis-, “(begin to) grow pale”; but kalas- “begin to shine, get light”; talat- > taltas- “begin to slip down”. These verbs were originally conjugated like TALAT-bases; but later their aorist and present forms coalesced in -sjā̆ (niñkwisjā̆); but their past retained the form -nsĭ/e (niñkwinsē). The majority of these verbs surviving in later Eldarin were from adjectival stems (PE22/135).

In Quenya the intervocalic s would be problematic in many verb tenses, since it would have become r. That may be why Tolkien said that the suffix was generally changed to -sya, though this would also develop into r as in the 3rd singular possessive suffix -rya < -syā. The net result would be aorist/present ✶ninkwisyā > ninquirya but past ✶ninkwinsē > ninquisse. There does, in fact, seem to be an example of such a verb in linguistic notes written between the 1st and 2nd edition of The Lord of the Rings. As it is described by Christopher Gilson:

The sheet also gives an assortment of verbal forms: mātare; amātielmi, mantelme; tankassen; tankarya, tankanse (?); tankatā, tankatalye, tankante and tanktealye, and the endings -arya; -tă, and -tā, of which the last is labelled “causative” (PE17/76).

The middle set of forms tankassen, tankarya, tankanse do indeed fit the probable pattern for past and present inceptives developed from primitive -sya for the verb tancarya-, perhaps meaning “become firm” (hat tip to Tamas Ferencz for pointing out this example). Thus the concept of this pattern for inceptive formation may have survived into the early 1960s. There is also indirect evidence for such inceptive formations in the Sindarin intransitive past tense of derived verbs (see the Sindarin grammar entry for details).

In Late Notes on Verb Structure (LVS) from 1969, Tolkien introduced a new mechanism for forming adjectival inceptives with the ancient suffix -tă, contrasted with transitive causatives formed with the ancient suffix -tā:

The denominative or adjectival verbs, causative & inceptive.

The transitive causatives were usually form[ed] with -tā, as were the causatives from verbal stems (as tultā- “send” < TUL “come”). So *tankātā- “to make firm, fix” > Q tankata.

The inceptives (become, grow, be made to be) used most frequently -tă, as *ninkwită- “grow white, whiten” > Q ninquitan, pa.t. ninquinte, fut. -itūva (ū < tău > tǝu > tū), perf. ininquitie.

no difference between aorist and pres (PE22/157).

In this new system, the stem forms of the transitive causatives (“make white”) and the intransitive inceptives (“become white”) were identical: ninquita- “make white; become white”. They can be distinguished only by whether or not the verb has an object: i atan ninquita “the man whitens (becomes white)”, i atan ninquita i ramba “the man whitens (makes white) the wall”. In the past tense, though, the two forms remain distinct, since the inceptives would use a half-strong past tense (ninquinte) whereas the causatives would use a weak past tense (ninquitáne): i atan ninquinte “the man whitened (became white)”, i atan ninquitáne i ramba “the man whitened (made white) the wall”.

Conceptual Development: Tolkien described several different strategies for forming inceptives over the years, especially for adjectival inceptives. The earliest of these was the verbal suffix -nta as in ᴹQ. ninqinta- “to grow pale”. This suffix was first mentioned in the Early Qenya Grammar of the 1920s (PE14/56) and reappeared in the Quenya Verbal System (QVS) from 1948 (PE22/114). These nta-inceptives were inflected as weak verbs. In QVS this suffix was a replacement for a rejected suffix tyō (PE22/114 note #82), probably a transient idea.

The other inceptives described in EVS2 were u-verbs and the suffixes lu, ru, nu, as described above (PE22/136). These were primarily (exclusively?) verbal inceptives and have no particular phonological difficulties. These are the best verbal inceptives available in the published corpus.

In the early 1950s Common Eldarin: Verb Structure Tolkien introduced the verbal and adjectival inceptives with suffixes lu, ru, nu vs -s (>> -rya) described above. Finally, Tolkien introduced yet another way to form adjectival inceptives in Late Notes on Verb Structure (LVS) from 1969 using -tă vs. causative -tā, also described above.

Neo-Quenya: For purposes of Neo-Quenya, I currently prefer the 1950s system described in EVS2 with verbal inceptives lu, ru, nu and adjectival inceptives -rya (past tense -nse) because they produce more distinct inceptive forms: Q. tancarya- “*become firm”, past tense tancanse vs. causative Q. tancata- “make firm”. The ancient inceptives with -s are also the best explanation of the Noldorin/Sindarin intransitive past tenses with -s. If, however, you prefer to use Tolkien “latest” system, then the 1969 system whereby causatives and adjectival inceptives are distinguished only in the past tense (inceptive ninquinte “became white” vs causative ninquitáne “made white”) is your best option.