Perhaps you should try some spelling variations:
toc sach » hoc sach, thuoc sach, nuoc sach
doc sach » hoc sach, nuoc sach, dac sac
cac nuoc » cap nuoc, hoc nuoc, dat nuoc
hieu ve » hieu va, hieu van
ky sang » ky nang, ky dang, ky tang
dau vi » dau ve, dau va, dat vi
va dau » va dao
ve cac » va cac, vien cac
vi cau » va cau, ve cau, di cau
ma hoi » mai hoi, xa hoi, me hoi

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