Perhaps you should try some spelling variations:
toc sach » hoc sach, doc sach, thuoc sach
sang mai » sang ma, sang tai, lang mai
cac nuoc » cap nuoc, hoc nuoc, dat nuoc
ky sang » ky nang, ky dang, ky tang
mai hoi » mai khoi, dai hoi, lai hoi
hieu ve » hieu van
doan vi » doan va, don vi, doan thi
sach ky » sac ky, sach y, sach ty
hieu va » lieu va, hieu qua
dan vi » don vi, dan va, gan vi

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 ky sang mai hoi. ((hieu ve) OR (hieu va)) cac nuoc va ((dan vi) or (doan vi)) cau thi~.
• If you are trying to use Boolean operators, they must be ALL CAPS: toc sach ky sang mai hoi. ((hieu ve) OR (hieu va)) cac nuoc va ((dan vi) OR (doan vi)) cau thi.
• The words AND, OR and NOT may confuse the search; try adding quotes: "toc sach ky sang mai hoi. ((hieu ve) OR (hieu va)) cac nuoc va ((dan vi) or (doan vi)) cau thi".
• Adding a wildcard symbol may retrieve word variants: toc sach ky sang mai hoi. ((hieu ve) OR (hieu va)) cac nuoc va ((dan vi) or (doan vi)) cau thi*.