* "One apple a day keeps the doctor away." (Here, "one" modifies "apple" to tell us how many apples we're talking about.)
* "He gave me one helpful suggestion." (Here, "one" modifies "helpful suggestion" to tell us how many suggestions he gave.)
* "There was one small problem with the plan." (Here, "one" modifies "small problem" to tell us how many problems there were.)
Remember, "one" functions as an adjective when it describes the quantity of something.
