Perhaps you should try some spelling variations:
nam thuc » nam thuoc, am thuc, nam thu
tu sach » thu sach, su sach, te sach
sach ky » sac ky, sach y, sach ty
va danh » va dinh, va dan, va hanh
va vanh » va van, va hanh, va anh
cua tre » cua te, cuu tre, cua triet
hieu va » lieu va, hieu qua
va nam » van nam, ha nam, va nay
o hai » co hai, so hai, o lai
o hoi » co hoi, o hoc, o hoa

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

• If you are trying to use Boolean operators, they must be ALL CAPS: tu sach ky nang ((o hoi) OR (o hai)). hieu va nam thuc ((va vanh) OR (va danh)) dai cua tre~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "tu sach ky nang ((o hoi) OR (o hai)). hieu va nam thuc ((va vanh) or (va danh)) dai cua tre~".
• Adding a wildcard symbol may retrieve word variants: tu sach ky nang ((o hoi) OR (o hai)). hieu va nam thuc ((va vanh) or (va danh)) dai cua tre*.