Can U Suggest M How Can I Find Substring

 hello all
 Plz can u suggest m how can i find substring like i want to show first 50 characters  and i dont want that my last 50th character should end with full word .. word should be complete  it may b 53 or 54 th character

for example :
m using  :
            substring(Remark,0,50) as Remark

and it returns :

 this is .................................. boo
but i want
 thi is ....................................book.

complete sentense plz suggest me how is this be don in SQL query.
thax alot