Perhaps you should try some spelling variations:
sach van » sach han, bach van, cach van
van hanh » van thanh, ban hanh, van anh
truc van » truc va, thuc van, truc ban
hanh de » hanh ve, hanh kem
hieu ve » hieu va, hieu van
cua tre » cua te, cuu tre, cua triet
sach va » sach xa, sach ha, sach dao
ve cau » ke cau, va cau, ve cac
de cua » le cua, te cua, dieu cua
o hoi » co hoi, o hai, o hoa
o hoc » co hoc, mo hoc, so hoc

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