Englishfor English speakers
a
Determinative
—
A is used when the following word could be any of a certain type.
Compare "A book I saw on the shelf" and "The book I gave you yesterday".
a
Noun
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A is the first letter of the alphabet.
The letter "a" comes before "b".
—
In some schools, an A is a very high grade.
Ron got an A on his earth science test.
tall
Adjective
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If something is tall, there is a long distance from the bottom to the top.
The man was so tall that he hit his head coming in the door.
There are too many tall buildings in Toronto.
one
Determinative
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(ordinal first) The number 1.
We have one nose and one mouth.
one
Noun
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The number 1.
One is the smallest whole number.
one
Pronoun
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One is a singular pronoun which means "someone" or "people". It is used more often in formal writing.
It is easy to see the difference if one looks closely enough.