The past (or passive) participle describes the condition you enter if you are exposed to the action of the corresponding verb (if someone sees you, you are seen; if someone kills you, you will thereafter be killed), or, in the case of some verbs, the condition you are in after having completed the action described by the verb (if you go, you will thereafter be gone). In Quenya, most past participles are derived from the corresponding verb with the ending -na or -ina. The past participle of car- "make" is carna "made"; the stem rac- means "break", while rácina is "broken" (if there is not a consonant cluster following the stem vowel, this vowel seems to be lengthened when the participal ending is added, like a > á in this case). If the stem ends in l, the ending -na is dissimilated to -da: mel- "love", melda "beloved" (Tolkien's gloss of the latter, "beloved, dear" rather than "loved", indicates that the distinction between adjectives and participles sometimes becomes blurred).
The past participle probably agrees in number with the noun it describes
(by final -a becoming -ë in the plural, just as with
normal adjectives), but the present participle does not change -la to
-lë as one might expect; it seems to be indeclinable (MC:222:
sisílala "wings shining", not **rámar sisílalë).
Perhaps this is to avoid confusion with the verbal noun ending -lë
"-ing" (as in Ainulindalë "the Music of the Ainur", literally