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