Perhaps you should try some spelling variations:
toc sach » hoc sach, thuoc sach, nuoc sach
doc sach » hoc sach, nuoc sach, dac sac
hieu van » hieu va, thien van, lieu va
nam thuc » nam thuoc, am thuc, nam thu
dau treo » dau tren, cau treo, dau theo
ky lang » ky dang, ky tang, tay lang
thuc va » thuc vat, thuc dan, thuc day
va dao » ca dao
dao va » dao van, tao va, do va

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 lang) OR (ky nang)) xa hoi. hieu van nam thuc va dao va dau treo~.
• If you are trying to use Boolean operators, they must be ALL CAPS: ((toc sach) OR (doc sach)) ((ky lang) OR (ky nang)) xa hoi. hieu van nam thuc va dao va dau treo.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((toc sach) or (doc sach)) ((ky lang) OR (ky nang)) xa hoi. hieu van nam thuc va dao va dau treo".
• Adding a wildcard symbol may retrieve word variants: ((toc sach) or (doc sach)) ((ky lang) OR (ky nang)) xa hoi. hieu van nam thuc va dao va dau treo*.