Perhaps you should try some spelling variations:
thanh dai » thanh hai, thanh tai, thanh dat
toc sach » hoc sach, thuoc sach, nuoc sach
va thanh » gia thanh, ban thanh, va tinh
nam truc » nam truoc, nam thuc, nam trung
hieu van » thien van, lieu va, hieu ve
doc sach » hoc sach, nuoc sach, dac sac
nang ma » tang ma, sang ma, nang cao
hieu va » lieu va, hieu qua
truc va » thuc va, thuc vat, trung va
cua tre » cua te, cuu tre, cua triet
ma hoi » mai hoi, xa hoi, ma hoa
y nang » ky nang, ly nang, v nang

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

• Performing a fuzzy search may retrieve terms with similar spellings: ((toc sach) or (doc sach)) y nang ma hoi. ((hieu van) OR (hieu va)) nam truc va thanh dai cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: ((toc sach) OR (doc sach)) y nang ma hoi. ((hieu van) OR (hieu va)) nam truc va thanh dai cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((toc sach) or (doc sach)) y nang ma hoi. ((hieu van) OR (hieu va)) nam truc va thanh dai cua tre".
• Adding a wildcard symbol may retrieve word variants: ((toc sach) or (doc sach)) y nang ma hoi. ((hieu van) OR (hieu va)) nam truc va thanh dai cua tre*.