Perhaps you should try some spelling variations:
doc sach » hoc sach, toc sach, nuoc sach
cac nuoc » cap nuoc, hoc nuoc, dat nuoc
hieu van » hieu va, thien van, lieu va
nuoc ca » nuoc cua, nuoc cap, nuoc va
van cac » van hac, van cau, van bac
ky tang » ky nang, ky dang, kho tang
ky sang » ky nang, ky dang, duy sang
ma hoi » mai hoi, xa hoi, ma hoa
dan vi » doan vi, don vi, gan vi
dan va » doan va, dan ca, ban va

You may be able to get more results by adjusting your search query.

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