Perhaps you should try some spelling variations:
ban hanh » ban hang
van hanh » van thanh, van anh, van ban
sach van » sach han, sach va, bach van
so sach » so sanh, bo sach, co sach
hieu ve » hieu va, hieu van
cua tre » cua te, cuu tre, cua triet
ve cau » ke cau, va cau, ve cac
de cua » le cua, te cua, dieu 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 ve cau truc ((van hanh) OR (ban hanh)) de 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 ve cau truc ((van hanh) OR (ban hanh)) de 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 ve cau truc ((van hanh) OR (ban hanh)) de cua tre".
• Adding a wildcard symbol may retrieve word variants: so sach van dong ((o hoi) or (o hoc)). hieu ve cau truc ((van hanh) OR (ban hanh)) de cua tre*.