Prepositions are link words beginning phrases.
They link the trunk of the sentence with:
the branches of time and/or place + manner + reason - adverbial phrase/s qualifying the verb
the descriptive branches - adjectival phrase - qualifying the noun
I read the book - main trunk of the sentence.
about dreams - adjectival phrase qualifying book.
It begins with the preposition about.
in the afternoon - adverbial phrase of time qualifying the verb read.
It begins with the preposition in.
beneath the tree - adverbial phrase of place qualifying the verb read.
It begins with the preposition beneath.
** prepositions are usually monosyllabic - one syllable
** Phrases do NOT contain a verb
** A group of 3 words usually = a phrase
for a full list of prepositions see
English Grammar 4U Online
(exercises are included)