Prepositions

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
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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.



NOTE

** prepositions are usually monosyllabic - one syllable
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** Phrases do NOT contain a verb
** A group of 3 words usually = a phrase

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for a full list of prepositions see
English Grammar 4U Online
(exercises are included)