+307.+ Many of the words that are used as prepositions are used also as
adverbs. It may be a little confusing to tell whether the word is an
adverb or a preposition, but if you will remember this simple rule you
will have no trouble:
+A preposition is always followed by either a noun or a pronoun as its
object, while an adverb never has an object.+
So when you find a word, that can be used either as a preposition or an
adverb, used alone in a sentence without an object, it is an adverb; but
if it is followed by an object, then it is a preposition. This brings
again to our minds the fundamental rule which we have laid down, that
every word is classified according to the work which it does in a
sentence. The work of a preposition is to show the relation between its
object and the word which that object modifies. So whenever a word is
used in this way it is a preposition. For example: _He went about his
business_.
Here, _about_ is a preposition and _business_ is its object. But in the
sentence, _He is able to be about_, _about_ is used as an adverb. It has
no object.
_He sailed before the mast._ Here, _before_ is a preposition introducing
the phrase _before the mast_, which modifies the verb _sailed_. But in
the sentence, _I told you that before_, _before_ is an adverb modifying
the verb _told_.
By applying this rule you can always readily determine whether the word
in question is an adverb or a preposition.