i want to ask how to use this phrase! ''one of the ''
And why this is a very poor sentence ?
--> As young people are the future pillars of society we must educate them well .
why this is wrong !
-->A police ran after the robber .
i hope u can help me ~ thanks ^^''
Welcome, Angel leung01.
 one of the
EX: One of the books on the table belongs to Max.
EX: I would like one of the blue hats, please.
EX: They want to use one of your computers.
 As young people are the future pillars of society we must educate them well.
When you use "as" or "because" as a conjunction, you are emphasizing the reason/cause, and so they should be used as connectors, like this,
EX: We must educate young people well, as they are the future pillars of society.
The connector "as" introduces a clause that answers the question Why? - Why must we educate young people well?
 A police ran after the robber.
Try, The police ran after the robber. The noun "police" is plural in number, so it isn't compatible with the indefinite article A, which is singular in number.
1. A book (singular, one book, indefinite/unknown)
2. The book (singular, one book, definite/known)
3. The books (plural, more than one book, definite/known)
4. A books (singular "A" and plural "books" are incompatible.
Hope that helps.