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
—
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.
quick
Adjective
—
Using very little time; moving far in little time.
You are able to run very quickly.
I will wash quickly.
He learns quickly.
quick
Adverb
—
Done rapidly.
He finished quick.
one
Determinative
—
(ordinal first) The number 1.
We have one nose and one mouth.
one
Noun
—
The number 1.
One is the smallest whole number.
one
Pronoun
—
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.