Perhaps you should try some spelling variations:
toc sach » hoc sach, doc sach, thuoc sach
cac nuoc » cap nuoc, hoc nuoc, dat nuoc
hieu ve » hieu va, hieu van
ky dang » ky nang, ky danh
ky sang » ky nang, ky tang, duy sang
ma hoi » mai hoi, xa hoi, ma hoa
ve cac » va cac, vien cac
dao vi » dao voi, dao va, dao li
dao ve » bao ve, dap ve, dau ve

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