Prepositions are interesting tools in our arsenals. They tell us how something connects or relates to an action or something else. Prepositional phrases can modify both nouns and verbs. For example, you can say:
The child on a plane
and
I play on a plane
In Sindarin, they carry out the same functions as in English.
When building a prepositional phrase you start with a preposition, since, after all, it is in the "pre-position." Sindarin has two types of prepositions, one that has "the" included in its meaning, and the other that doesn't.
Erin gond on the rock
Po i 'ond on the rock
Prepositional phrases can behave like adjectives, following the nouns they modify, appearing after any adjectives that may be attached to the noun. When they are carrying out this role, the preposition is lenited. Prepositions can't become plural to match a noun that they follow, however.
Hên bo 'ond vi dhuin vi dawar - A child on a rock in a river in a forest.
Prepositional phrases can also behave like adverbs, following the verbs they modify, and following any other adverbs, pronouns, and other verb-adjacent words connected to the verbs. If it directly follows the verb, the preposition is lenited, just like an adverb would be.
Hên laba bo 'ond. - A child jumps on a rock.