Perhaps you should try some spelling variations:
viet sach » viet anh, dien sach, viet tech
sach nuoc » sach duoc, cac nuoc, bach tuoc
hieu viet » sieu viet, hieu viec, lieu viet
va xanh » va danh, va hanh, va banh
vu sach » vu bach, su sach, vu khach
sang ma » tang ma, nang ma, sang tao
cua tre » cua te, cuu tre, cua triet
va anh » van anh, va danh, va hanh
di cua » dai cua, doi cua, de cua

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

• Performing a fuzzy search may retrieve terms with similar spellings: vu sach y ((sang ma) or (nang cao)) hoi. hieu viet sach nuoc ((va xanh) OR (va anh)) di cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: vu sach y ((sang ma) OR (nang cao)) hoi. hieu viet sach nuoc ((va xanh) OR (va anh)) di cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "vu sach y ((sang ma) or (nang cao)) hoi. hieu viet sach nuoc ((va xanh) OR (va anh)) di cua tre".
• Adding a wildcard symbol may retrieve word variants: vu sach y ((sang ma) or (nang cao)) hoi. hieu viet sach nuoc ((va xanh) OR (va anh)) di cua tre*.