The accusative form in Quenya is used for the object of verbs and is unmarked in “modern” Quenya (Tarquesta). According to the Plotz letter, there was a distinct accusative form in Classical Quenya (Parmaquesta), marked with a long final vowel in the singular, and by the use of an -i plural for vocalic nouns in the plural:
- Singular accusative: ciryā, lassē (vs. nominative cirya, lasse)
- Plural accusative: ciryai (vs. nominative ciryar)
We don’t know how (or whether) the accusative was marked for dual or consonantal nouns (but see below).
Presumably the accusative form was also used for the object of prepositions, as it is with English object pronouns: “I saw him, I gave it to him”. It is hard to determine for certain, however, since by Tarquesta there was no distinction between the nominative and the accusative. There was a Common Eldarin objective form, which in one place Tolkien said was used for both verbal objects and the object of prepositions (PE21/67), so I think it likely the Quenya accusative was used the same way. This is largely a moot question in Tarquesta, but is relevant if you are trying to imitate Parmaquesta grammar.
As noted above, Common Eldarin had an objective “inflection” that was simply the unmarked form of the noun (PE21/67, 75). It is not entirely clear how this form evolved into the Parmaquesta accusative. Perhaps in vocalic nouns, the accusative form was more “conservative” and preserved long vowels that were shortened in the nominative, as well as preserving the more ancient i-plurals which had become the r-plurals in the nomitative (see plural nouns). Assuming this is correct, then likely Parmaquesta would not have had a distinct accusative form for consonantal nouns, though dual accusatives might have shown a long final -ū and e-nouns might have shown long final -ī in the accusative plural.
Conceptual Development: In the Early Qenya Grammar of the 1920s, the accusative form was marked with a -t in the singular and -nt [-(l)int] in the plural (PE14/43, 73), though in the manuscript version Tolkien said the -nt marker was poetic, and the normal accusative plural marker was -n: -(l)in (PE14/43). In a set of Qenya Declensions charts from later in the 1920s, Tolkien decided that the accusative and base forms of the noun were the same, so that the accusative was unmarked with the exception of singular consonantal nouns where it was marked by -a (PE16/111-115).
Tolkien retained this system in the more lengthy Declension of Nouns from the early 1930s (PE21/1-41), with the caveat that vocalic declensions ending in -i or -u also added -a in the accusative (PE21/14-15). In later declension charts signs the system described in Plotz began to emerge, with the retention i-plurals in the accusative (PE21/50, 53), and long -í in the accusative plural of e-nouns (PE21/53). Tolkien’s notes and use of parenthesis indicate these distinct accusative forms were archaic, just as they were in Plotz.