Prepositions are linking words in a sentence. We use prepositions to explain where things are in time or space. Prepositions tell us where something is (for example, beside, under, on, against, beneath or over) or when something is happening (for example: until, during, after, before or more specifically ‘on Christmas Day’, ‘at twelve o’clock’ or ‘in August’).
Prepositions usually sit before nouns (or pronouns) to shows the noun’s (or pronoun’s) relationship to another word in the sentence.
source: theschoolrun.com
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