Perhaps you should try some spelling variations:
sang mai » sang ma, sang tai, lang mai
cac nuoc » cap nuoc, hoc nuoc, dat nuoc
cau theo » cau thi, cau tao
mai hoi » mai khoi, dai hoi, lai hoi
hieu ve » hieu va, hieu van
sach ky » sac ky, sach ty, sach kem
doan vi » doan va, don vi, doan thi
dan vi » don vi, dan va, gan vi
ve cac » va cac, vien cac

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