Quenya does not have an indefinite article, but it does have a set of indefinite pronouns: mo “some (indefinite) person” and ma “some (indefinite) thing”:

mo, indefinite personal pronoun “somebody, one”.
ma, neuter personal pronoun “something, a thing”.

For the indefinite mo the inclusive 1.pl.= “we, you and I (and others associated)” was often used, espec. colloquially, like E. “you” (notes from the late 1960s, PE22/154).

The personal indefinite pronoun mo is used as an indefinite subject in much the way that English sometimes uses the word “one”: alasaila ná lá kare tai mo nave mára “it is unwise not to do what one judges good” (VT42/34). This use of indefinite subjects sometimes serves a function similar to English passive voice (PE22/154 note #57), for example: mo quete i Eldar vanime nér “one says the Elves were beautiful” or in more colloquial English “it is said the Elves were beautiful”. A suffixal form -mo frequently functions as an agental suffix: onótimo “reckoner” (MR/49), more literally “count-up-person” as suggested by Carl Hostetter and Patrick Wynne (VT34/30).

We have fewer examples of how the neuter indefinite pronoun ma was used, and its use may be limited to objects, not subjects, such as: yé mána ma “what a good thing” (PE17/162). This is because the neuter indefinite pronoun is identical in form to the interrogative particle ma, and if it were to appear at the beginning of a sentence, the phrase would probably be mistaken as a question. There is no such confusion with personal interrogatives, because “who” was man or mamo, not mo. For example, “something must be done” might be mauya mon care ma = “it is needed that someone do something” in Quenya.

Origins of indefinite pronouns: The personal indefinite mo was derived from primitive ✶mō which simply meant “person” (VT47/35), while the neuter indefinite ma was probably related to and derived from the interrogative root √MA.

Conceptual Development: The Early Qenya Grammar (EQG) has an indefinite article in the form of the suffix ᴱQ. -(u)ma (PE14/42, 71), almost certainly the precursor to the indefinite/interrogative ma as suggested by Christopher Gilson (PE17/68). We don’t know what the indefinite pronouns were at this stage, however, and it seems that impersonal/indefinite subjects were simply omitted entirely in EQG:

Note the neuter is never used as an impersonal subject: there is no prefix used at all in that case, as: uqe “it rains”; tiqe “it thaws” (PE14/56).

The indefinite suffix ᴹQ. -(u)ma “a certain” resurfaced in Demonstrative, Relative, and Correlative Stems (DRC) from 1948 (PE23/105), and in that document ᴹQ. uma- was also a prefix meaning “some” as in ᴹQ. umallume “some time” (PE23/110). The indefinite pronoun in this document was ᴹQ. ane “somebody, something”, however, based on the indefinite subject pronoun ᴹQ. a (PE23/103).

The indefinite pronoun a also appeared in Quendian & Common Eldarin Verbal Structure from the 1940s as well as other documents documents in this period describing pronouns (PE22/94; PE23/75, 88, 93). This pronoun also appeared in the contemporaneous Quenya Verbal System, in constructions resembling passive voice: i·nér né raiqa ar sí aphastat “the man was angry but now is in good humor” where a-phasta = “it pleases (him)” (PE22/124); masse akime aldar “where are the trees” more literally “where does one find trees” (PE22/125).

These same 1940s documents had a third person singular pronoun ᴹQ. e “he, she, it” (PE23/75, 88, 93), which in later writings became se. The pronoun e briefly reappeared in notes from the late 1960s as an indefinite personal pronoun, but was soon changed to mo (PE22/154 notes #57, 68). For the most part, in the 1950s and 60s Tolkien seem to use mo/ma for indefinite pronouns, as discussed above.

Neo-Quenya: For purposes of Neo-Quenya, I would use mo “someone” and ma “something” for indefinite pronouns, but for other indefinites I would use uma- “some [singular]” and li(n)- “many [plural]” as described in DRC. See the entry on Quenya correlatives for further information.