In English, indefinite pronouns begin with "some-," "any-," "every-," and "no-."
In Sindarin, use normal lenition rules for them; treat them like normal nouns. They don't have a prefix for "some-" or "any-", but the prefix for "every-" is il- and "no-" is al-.
The pronoun for "one/person/somebody/someone" is pen. Its opposite, "no one," is *alben. "Everyone" is *ilphen.
Cenin ben. - I see someone.
Alben len gweriant. - No one betrayed you.
Ilphen istar. - Everyone knows.
The pronoun for "thing/something" is nad. Its opposite, "nothing," is *alnad. "Everything" is *ilnad.
Mado nad egor 'wanno! - Eat something or die!
Ach evennin alnad! - But I ate nothing!
Ilnad *rangen. - Everything is broken.
The pronoun for "place/someplace/somewhere" is sad. Its opposite, "nowhere," is *alhad. "Everywhere" is *ilhad.
Nidhin toled na had laug. - I will go to somewhere warm.
Baren alhad. - My home is nowhere.
Trevennin ilhad. - I've traveled everywhere.
A reflexive pronoun is a pronoun that reflects the subject of the sentence. In English, reflexive pronouns end in "-self." In Sindarin, the reflexive pronouns im, anim, and �n are not marked by person or number, so you can use them to refer to any subject. Im could mean "myself, ourselves, yourself, yourselves, itself, herself, himself, themself, themselves" depending on the context it's in. Anim is just the dative version of im. Likewise, �n could mean "my own, our own, your own, y'all's own, its own, her own, his own, their own."
Im can be used in four different ways.
1. It can be used as the object of a verb. When it's being used this way, put it after the verb, like any other noun, just like the other indefinite pronouns are used.
Ristannen im. - I cut myself.
Harnol im. - You hurt yourself.
Amathon a Reinor *law eglerianner im. - Amathon and Reinor didn't glorify themselves.
2. It can be used with a preposition like any other pronoun or noun.
�nen ant anim. - I gave a gift to myself.
Peliant laiss oh im. - It spread leaves around itself.
3. It can be used to emphasize the subject of the sentence, when used alongside the subject of the sentence. It's placed directly before the subject. If the subject is a pronominal suffix, then it goes in the place that a noun subject would usually go.
Im Glaewen di nestatha. - Glaewen herself will heal them.
Im degin yrch. - I myself slay orcs.
Im peniathol i banas? - Would you yourself please fix the floor?
Im Triwath peniathol i banas? - Would you Triwath please fix the floor?
4. It's used to introduce yourself, since it basically means "self." Don't mutate the name, because it is in the vocative case and you want people to know your non-mutated name.
Im Nimmiel. - I'm Nimmiel.
Treat �n as any other possessive pronoun. It is especially useful in the 3rd person, since the 3rd person possessive pronoun is the same for the singular and plural.
Ivreth sogant i n�n �n. - Ivreth drank her own water.
If you used t�n instead, it'd mean, "Ivreth drank his/her/its/their water," implying that she drank someone else's water, instead of her own!