413. One for A(or An)
(X) Adam found one ring in the street.
(O) Adam found a ring in the street.
Don't use the numeral "one" instead of the indefinite article "a" or "an".
Use "one" only where the number is emphatic: He gave me one book instead of two.
414. Less for Fewer
(X) They have less books than I have.
(O) They have fewer books than I have.
"Less" denotes amount, quantity, value, or degree. "Fewer" denotes number.
We may have "less water, less food, less money, less education",
but "fewer books, fewer letters, fewer friends".
Note: We say less than (five, six, etc.) dollars because the dollars are
considered as "a sum of money" and "not as a number of coins".
415. This for That
(X) Look at this dog across the street!
(O) Look at that dog across the street!
"This" is used to indicate something physically close to the speaker.
In the case of abstract things we use "this" for things which are most immediately present.
This is a lovely song! / I'll help you do it this time.
When we talk about more than one thing we use "this" for the closer or more immediate and
"that" for the further away or more remote in time.
If we are only talking about one thing we usually use "that".
What's that noise? / That's a nice coat! / Don't do that!