In addition to the active participle (-(i)la) and the passive participle (-ina), Tolkien discussed a third suffix -nwa, which he variously labeled a “passive suffix”, a “perfective adjective” or a “perfective participle”.

The perfective participle. This was formed with -nwa. It originally was not passive or active but denoted the completion of the action denoted by the verb. It is found in words outside the verbal system, as from stem vā- “go”: vanwa “gone for good, departed (dead, lost)”. Added to the aorist stem, or in weak verbs the aorist-present, it retains this function: karinwa “made, finished, done, completed”; tulinwa “arrived, come, now at hand”; alanwa “full-grown, mature, adult”; istanwa “known, acknowledged, certain”.* [In a footnote: No confusion arises between “weak presents” and similarly formed causatives since where the forms coalesce so do the senses: ortanwa < órt(a) “rise”, ortanwa < ortā́ “raise” = “having risen on high/upraised on high” = “exalted”] With a {past >>} perfect stem -nwa forms the perf. part. passive. This is not used as part of verbal system (Quenya Verbal System, late 1940s, PE22/106).

There was also a perfective adjective made with -nwa which expressed completion of the action (transitive or intransitive as the case might be). This is seen notably in vanwa (√BA “go, proceed”) = “gone for good, departed (dead, lost)”. In ordinary verbs it is usually added to the aorist stem: karinwa “fully made, completed”; tulinwa “arrived, now at hand”; alanwa “fully grown, adult, mature”; istanwa “known, generally recognized” (Quenya Verbal System, late 1940s, PE22/112).

vanwa, adj. “gone, past, lost”, irregular in having -nwa, usually a passive suffix (from Notes on Galadriel’s Song, NGS, late 1950s or early 1960s, PE17/63).

-na, adjective, simplest form of participle (no longer part of verbal conjugation) ... As part of conj[ugation] -ina, aorist, is used ... Simple past participle passive, kari-nwa, adj. -ina, after vowel stems -nwa, sinwa, sīna “known, certain, ascertained”. After intransitives often = participle active, va-nwa. This has a past form kárienwa (rare) (notes from 1967, PE17/68).

In the Quenya Verbal System (QVS) of the 1940s it seems -nwa “is not used as part of verbal system”, and is simply an ancient adjective suffix applied to verbal roots. This suffix has the sense of an “completed action” and can be used with both transitive and intransitive verbs. It is added to the verb stem for verbs ending in a vowel, and as -inwa to basic verbs ending in a consonant.

These -nwa adjectives behave like the English past participle (when used adjectivally), which likewise can be used both transitively or intransitively. When used with a transitive verb, the -nwa adjectives are functionally very similar to the passive participle, where the modified noun is the object of the action: perfective karinwa “made, finished, done, completed” (PE22/106) vs. passive karina “made” (PE22/111). When used with an intransitive verb, they are functionally similar to the past/perfect active participle: perfective tulinwa “arrived, now at hand” vs. past/perfect active túlielya (1940s) or túliéla (1960s) “*arrived, having come”.

Although not “part of the verbal system”, in QVS the suffix -nwa was used to form the past/perfect passive participle:

The equivalent passive participles were made only from the perfect stem (without augment) + the suffix -nwa. kárienwa “having been made”; mátienwa “having been eaten” (beside mantienwa) etc. ... Since nwa is not specifically passive (see above) it is sometimes found with intransitive verbs in place of [active perfect participle] -elya: túlienwa “having arrived”. This is mostly archaic or † [poetic] (PE22/109).

This formation was mentioned again in the late 1960s: “This has a past form kárienwa (rare)” (PE17/68). The use of -nwa over -ina to form the past/perfect passive participle was probably due to (a) the inherently perfective nature of -nwa and (b) the difficulty of combining the passive participle suffix -ina with the perfect ending -ie.

Neo-Quenya: Some older Neo-Quenya courses suggest that -nwa is an alternate way of forming the passive participle, but I would reserve -ina for passive participles. I would treat -nwa as a distinct suffix used to form perfective adjectives from verbs, with a passive sense when used with transitive verbs, but a past/perfect sense when used with intransitive verbs. This is complicated, though, by the fact that passive participles can also sometimes be used with intransitive verbs, where they take on a similar past sense:

aorist passive adj. karina. also used with intrans[itives] as kwal- “die”, qualina “dead” (PE22/152).

I think all these forms can coexist, however, with slightly different connotations. In particular, with -nwa having a specific “perfective” sense of a completed action or process, and being used only adjectivally, not participially.