noun
refers to the number 1 or a single person or thing
adjective
used to refer to a single person or thing, or to emphasize that there is only one
pronoun
used to refer to a person or thing previously mentioned or easily identified
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In this experiment, one must carefully follow the instructions to achieve accurate results.
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If you're looking for a good restaurant, that one over there is really popular.
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I can't believe that one got away with cheating on the test.
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Sometimes in life, we have to let go of the old to make room for the new, like shedding one's skin.