Perhaps you should try some spelling variations:
hieu cac » hieu luc, hieu va, hieu van
hien cac » thien cac, vien cac, dien cac
doan vi » doan va, don vi, doan thi
sach ky » sac ky, sach y, sach ty
su sach » so sach, vu sach
ky tang » ky nang, ky dang, kho tang
tang xa » trang xa, tang ma, lang xa
cua tre » cua te, cuu tre, cua triet
dan vi » don vi, dan va, gan vi

You may be able to get more results by adjusting your search query.

• Performing a fuzzy search may retrieve terms with similar spellings: su sach ky tang xa hoi. hieu ((hien cac) OR (hieu cac)) tu va ((dan vi) or (doan vi)) cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: su sach ky tang xa hoi. hieu ((hien cac) OR (hieu cac)) tu va ((dan vi) OR (doan vi)) cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "su sach ky tang xa hoi. hieu ((hien cac) OR (hieu cac)) tu va ((dan vi) or (doan vi)) cua tre".
• Adding a wildcard symbol may retrieve word variants: su sach ky tang xa hoi. hieu ((hien cac) OR (hieu cac)) tu va ((dan vi) or (doan vi)) cua tre*.