The Quenya genitive is formed with the suffix -o and is roughly equivalent to the English preposition “of”. Tolkien talked about the circumstances in which the genitive would be used at some length in his Quenya and Eldar essay written in 1960s, mostly comparing it to the possessive case (WJ/368-369):
The Quenya dative is used for the indirect object of a phrase and is formed using the suffix -n. The indirect object is the recipient or beneficiary of an action, as opposed to the direct object which is the immediate target. In English, the indirect object comes immediately before the direct object in a sentence: “I give you the knife”, “I wish you well”. Alternately, English can indicate the indirect object with prepositions like “to” or “for”: “I give the knife to you”, “I wish happiness for you”.
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 for singular vocalic nouns, and by the use of an -i plural for vocalic nouns in the plural:
Totally off topic, but Sir Ian McKellen is 80 and tours the world with a one-man show celebrating his long career - I saw him the other day in London, and (naturally) he started the show by reciting the Bridge of Khazad-dûm scene from The Fellowship :) and afterwards he produced the actual Glamdring he used as a prop in the films and brandished it around the stage.
There was much more in the show of course, and he is a lovely man. A great night.
Like most languages, Quenya distinguishes between singular (one) and plural (multiple) nouns. Quenya has two special plural forms, the dual (for pairs) and the partitive plural (for portions of groups = “some”). This entry discusses the general plural, which is used for all the other forms of noun plurality.
A couple of months ago @Paul Strack posted a short but neat analysis of the attested Quenya particles of uncertainty, their possible shades of meaning and use. Since Discord is a platform we cannot link to, I am copying the text here, for further discussion, and to preserve it for posterity:
ma = interrogative/indefinite (probably true)
qui = neutral hypothetical (may or may not be true)
ai/ce = strong hypothetical (probably not true)