Perhaps you should try some spelling variations:
toc sach » hoc sach, doc sach, thuoc sach
hieu van » hieu va, thien van, lieu va
hanh dan » thanh dan, hanh san, canh dan
su triet » su viet, su tien, ve triet
thuc va » thuc vat, thuc dan, thuc day
va hanh » van hanh, va thanh, va danh
van am » van nam, van lam, van dam
dan su » dang su
o hoc » mo hoc, so hoc, y hoc

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

• Performing a fuzzy search may retrieve terms with similar spellings: ((te sach) or (toc sach)) ky nang ((o hoc) OR (co hoc)). hieu van am thuc va hanh dan su triet~.
• If you are trying to use Boolean operators, they must be ALL CAPS: ((te sach) OR (toc sach)) ky nang ((o hoc) OR (co hoc)). hieu van am thuc va hanh dan su triet.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((te sach) or (toc sach)) ky nang ((o hoc) OR (co hoc)). hieu van am thuc va hanh dan su triet".
• Adding a wildcard symbol may retrieve word variants: ((te sach) or (toc sach)) ky nang ((o hoc) OR (co hoc)). hieu van am thuc va hanh dan su triet*.