Perhaps you should try some spelling variations:
nam thuc » nam thuoc, am thuc, nam tu
hanh tai » thanh tai, hang tai, han tai
hieu van » thien van, lieu va, hieu ve
tu sach » thu sach, su sach, te sach
tai cua » thai cua, tai cuc, tri cua
xa hanh » xa thanh, xa khanh, va hanh
hieu va » lieu va, hieu qua
tang ma » mang may, tang van
sach ky » sac ky, sach y, sach ty
ky tang » ky nang, ky dang, kho tang
cua tre » cua te, cuu tre, cua triet
ma hoi » mai hoi, xa hoi, ma hoa

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