Verbs can become Participles. They're still slightly verb-like, as they can occasionally have direct objects, but they also behave very similarly to adjectives - following nouns, being lenited, and some of them become plural with their nouns.
The Past Participle is an adjective that describes something that was done to its noun.
Past Participles can become plural.
For the A-verb, simply conjugate it in the first person past tense.
eitha- insult → eithannen insulted
cuia- live → cuiassen lived
For the I-verb, add -nen to it. This has a 2-step process.
1. First add the -n like you're making the verb into the past tense. The only difference is that verbs ending in continuants aren't excluded. They change as follows:
2. Then add -en.
gad- catch → gannen caught
dag- slay → dangen slain
hal- lift → hallen lifted
dew- fail → dewnen failed
fir- die → firnen dead
laf- lick → lammen licked
*ceth- search → *cethen searched
In English, we use past participles to make the "past perfect," a past tense that describes an action that was completely finished sometime before being spoken of, and that the effects of the actions' completion are still relevant. Sindarin doesn't appear to have a perfect past tense, except perhaps for intransitive verbs. If you find yourself translating "to have [past participle]" into Sindarin, you need the simple past instead.
Passive voice in English is built from [to be] + past participle. It makes the noun that is acted upon into the subject, by describing its state as something that had an action done to it.
Passive voice is used in several ways - it hides the doer of the action. This is especially true of Sindarin, which doesn't have a way to add "by someone" to the sentence, at least not that we know of yet. It also brings the emphasis of the sentence onto who or what the action was done to.
In Sindarin, passive voice is made the same way it is in English:
noun [to be] past participle
"To be" is left out of the sentence, and the past participle isn't mutated. It is, however, made plural or singular to match the subject.
Ada dangen. - Daddy was slain.
I chenneth *rangen. - The window was broken.
Ti berthennin. - They are doomed.
Use the intransitive verbs to make a past perfect tense - describing an action that not only happen in the past, but has had some time pass after the action was completed. The simple past describes an action that could have happened only moments before.
This syntax only works for the intransitive verbs. English used to have a very similar syntax in it, but it was eventually replaced with "have+past participle." In archaic English texts, you can find sentences like "I am gone to the town" instead of "I have gone to the town."
I annon dannen. - The gate has fallen.
I ch�n galassen. - The child has grown.
First, turn the direct object of the verb into the topic of the phrase. Then the indirect object is made into the subject, and the verb is a past participle.
[Subject] [verb] [direct object] [indirect object] → [direct object] [indirect object] [past participle]
Reinor aun *gorf an 'Waeren. → *Corf an 'Waeren annen.
Reinor gave a ring to Gwaeren. → A ring was given to Gwaeren.
Or with an dropped:
Reinor aun *gorf Gwaeren. → *Corf Gwaeren annen.
Reinor gave Gwaeren a ring. → A ring was given to Gwaeren.
This is also useful for introductions, because it means that the name wouldn't be lenited.
[someone] [a name] estannen
Aranyo Reinor estannen be Edhellen - Aranyo is called Reinor in Sindarin.
I ch� hen Huan estannen. - This dog is called Huan.
Im Haengil estannen. - I'm called Haengil.