The same word may be sometimes one part of speech, sometimes another. The meaning of a word in the sentence determines to what part of speech it belongs.

Words of entirely separate origin, meaning, and use sometimes look and sound alike: as in “The minstrel sang a plaintive lay,” and “He lay on the ground.”

But the following examples show that the same word may have more than one kind of grammatical office (or function). It is the meaning which we give to a word in the sentence that determines its classification as a part of speech.

So, let’s take these examples:

NounsAdjectives
Rubber comes from South America.This wheel has a rubber tire.
That brick is yellow.Here is a brick house.
The rich have a grave responsibility.rich merchant lives here.
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The first two examples show how words that are commonly nouns may be used as adjectives; the third shows how words that are commonly adjectives may be used as nouns.

NounsVerbs
Hear the wash of the tide.Wash those windows.
Give me a stamp.Stamp this envelope.
It is the call of the sea.Ye call me chief.

Other examples are: 

  • act,
  • address,
  • ally,
  • answer,
  • boast,
  • care,
  • cause,
  • close,
  • defeat,
  • doubt,
  • drop,
  • heap,
  • hope,
  • mark,
  • offer,
  • pile,
  • place,
  • rest,
  • rule,
  • sail,
  • shape,
  • sleep,
  • spur,
  • test,
  • watch,
  • wound.
AdjectivesAdverbs
That is a fast boat.The snow is melting fast.
Draw a straight line.The arrow flew straight.
Early comers get good seats.Tom awoke early.
AdjectivesPronouns
This man looks unhappy.This is the sergeant.
That book is a dictionary.That is a kangaroo.
Each day brings its opportunity.I received a dollar from each.
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For further study of this class of words, see part of speech.

AdverbsPrepositions
Jill came tumbling after.He returned after the accident.
We went below.Below us lay the valley.
The weeds sprang up.We walked up the hill.

Other examples are: 

  • aboard,
  • before,
  • beyond,
  • down,
  • inside,
  • underneath.

Miscellaneous examples of variation are the following:

Noun.The calm lasted for three days.
Adjective.Calm words show quiet minds.
Verb.rne your angry friend.

Other examples are: 

  • iron,
  • stone,
  • paper,
  • sugar,
  • salt,
  • bark,
  • quiet,
  • black,
  • light,
  • head,
  • wet,
  • round,
  • square,
  • winter,
  • spring.