Perhaps you should try some spelling variations:
hieu van » hieu va, thien van, lieu va
sach van » sach han, sach va, bach van
dieu cua » dieu tra
van hanh » van thanh, ban hanh, van anh
truc van » truc va, thuc van, truc ban
van cau » van dau, van hau, van chat
so sach » so sanh, bo sach, co sach
de cua » le cua, te cua
o hoc » co hoc, mo hoc, so hoc
o hoi » co hoi, o hai, o hoa

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

• Performing a fuzzy search may retrieve terms with similar spellings: so sach van dong ((o hoi) or (o hoc)). hieu van cau truc van hanh ((de cua) OR (dieu cua)) tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: so sach van dong ((o hoi) OR (o hoc)). hieu van cau truc van hanh ((de cua) OR (dieu cua)) tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "so sach van dong ((o hoi) or (o hoc)). hieu van cau truc van hanh ((de cua) OR (dieu cua)) tre".
• Adding a wildcard symbol may retrieve word variants: so sach van dong ((o hoi) or (o hoc)). hieu van cau truc van hanh ((de cua) OR (dieu cua)) tre*.