's

[ehsanshekari]

in "teacher's book" 's is used but it isn't used in "student book"
or other examples like "john's book" "cigarette ash"
why sometimes 's is used and sometimes it isn't ?

 

[Anglika]

Sometimes it is a possessive [ teacher's book = the book belonging to the teacher; John's book - the book belonging to John ]; sometimes is is an adjective [cigarette ash = ash from a cigarette; student book = book for use by students].