A class of words used with nouns and other words to form Prepositional Phrases. Prepositions form a small group of, generally, small words. The commonest are:
About above across after against
Along among around as at
Before behind below beneath beside
During except for from in
Upon with within without
There are also two-, three-, and four words prepositions:
Along with apart from as well as away from
Because of close to except for in front of
In the face of instead of next to on to
On top of out of owing to up to
Prepositions come before:
 A Noun:
Below groud.
 A Pronoun:
After me.
 A Verbal Noun:
Without leaving.
 A Noun Phrase:
During the last month.
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