We have verbs that are labeled as impersonal in Sindarin, but no examples of them being used. There are examples in Quenya and Tolkien's earlier drafts of Elven languages, indicating that his construction for them hadn't changed. Using this syntax is risky, but it's common enough in Neo-Sindarin circles to include it.
An impersonal verb is one that doesn't have a subject, but can have an object. In English, impersonal verbs still have subjects, like "it's raining," and "it's snowing," but these subjects don't have any meaning, they're just placeholders, because our syntax demands that something be there. Then, what is doing the action? For English, in our modern era, it appears that it's nature doing the action of "raining" or "snowing." In Sindarin, based on Tolkien's world, we can say that it's the Ainur.
Because of the Ainur and the roles that the Elves believe that they hold on their world, things like sudden, apparently uncaused changes in emotion or mood are attributed to the Ainur, as are dreams, visions, and hallucinations. Having an impulse or having a dream aren't things that you do, they are done to you.
Here are the known or guessed verbs that can only be used impersonally:
These are some of the verbs can be used as impersonal verbs and as regular verbs. Using them with impersonal syntax emphasizes that the source of the action is mysterious, unknown, or likely supernatural in nature.
The syntax of impersonal verbs is quite similar to the syntax of a command. The impersonal verb goes to the beginning of the sentence, and the indirect object follows. These verbs can't have direct objects, only indirect objects, so you can use the long or short dative as you please.
Pessa ammen. - We are being concerned/affected/moved/troubled/changed.
Pessa ven. - We are being concerned/affected/moved/troubled/changed.