Perhaps you should try some spelling variations:
toc sach » hoc sach, doc sach, thuoc sach
mai khoi » mai khoa, tai khoa, cac khoi
cac nuoc » cap nuoc, hoc nuoc, dat nuoc
doan vi » doan va, don vi, doan thi
sach ky » sac ky, sach y, sach ty
hieu ve » hieu va, hieu van
ky sang » ky nang, ky dang, ky tang
mai hoi » dai hoi, lai hoi, mai doi
ve cac » va cac, vien cac
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) OR (mai khoi)). hieu ve 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) OR (mai khoi)). hieu ve 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) OR (mai khoi)). hieu ve cac nuoc va ((dan vi) or (doan vi)) cau thi".
• Adding a wildcard symbol may retrieve word variants: toc sach ky sang ((mai hoi) OR (mai khoi)). hieu ve cac nuoc va ((dan vi) or (doan vi)) cau thi*.