Perhaps you should try some spelling variations:
hieu luc » hieu luat, hieu qua
hieu cac » hieu va, hieu van, vien cac
tu sach » thu sach, su sach, te sach
va dinh » van dinh, va kinh, va tinh
cua tre » cua te, cuu tre, cua triet
sach ty » sach y, sach tu, sach thu
nang xa » nang x, lang xa, nang ma
ty nang » ky nang, ty xang, ty hang
va dan » ca dan, va dao, va dau

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

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