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.
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). Tolkien outlined this distinction in Late Notes on Verb Structure from 1969:
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).
The net result is that in many cases 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 nér ninquita “the man whitens (becomes white)”, i nér 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 (ninquitane): i nér ninquinte “the man whitened (became white)”, i nér ninquitane 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 the 1940s (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.
In the 1950s Common Eldarin: Verb Structure (EVS2) he gave two different ways to form inceptives: with consonantal suffixal -s (PE22/135) and the verbal suffixes lu, ru, nu (PE22/136). Of the first he 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. The relationships between the tenses would be very obscure, and it is possible these -rya inceptives would degrade into weak verbs.
The other inceptive described EVS2 were u-verbs and the suffix 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.
Finally, Tolkien introduced yet another way to form adjectival inceptives in Late Notes on Verb Structure from 1969, as described above: using formative -tă for inceptives, as opposed to causative -tā. I recommend this pattern for the purposes of Neo-Quenya, though it does mean the inceptives are not very distinct from the causatives.