The philosopher Aristotle and later scientists studied animals and
classified them according to what they have in common. For example,
eagles, robins and sparrows are kinds of birds; sharks, salmon and tuna
are kinds of fish; and dogs, horses and elephants are kinds of mammals.
Aristotle and others also studied language and classified words according
to what they have in common. We usually use 8 categories or parts of
speech to classify all the words we use in English.This classification is
not perfect. Sometimes it is hard to tell which category a word belongs in.
The same word may belong in different categories depending on how it is
used. There may be better ways to classify Englishthan by using the 8
parts of speech. But this classification has been used for a long time and
many grammar books use it, so it is easier to keep on using it. It is
possible to speak or learn a language without knowing the parts of
speech, but for most of us, knowing about parts of speech makes things
easier.