Perhaps you should try some spelling variations:
thu sach » tu sach, thuat sach, thuoc sach
sach ky » sac ky, sach y, sach ty
qua cac » cua cac
cac duc » cac du, cao duc, cac dac
hai cua » thai cua, dai cua, hoi cua
cua tre » cua te, cuu tre, cua triet
va dao » ca dao
o hoi » co hoi, o hoc, o hai

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

• Performing a fuzzy search may retrieve terms with similar spellings: thu sach ky nang ((x ho) or (o hoi)). hieu qua cac duc ((va dau) OR (va dao)) hai cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: thu sach ky nang ((x ho) OR (o hoi)). hieu qua cac duc ((va dau) OR (va dao)) hai cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "thu sach ky nang ((x ho) or (o hoi)). hieu qua cac duc ((va dau) OR (va dao)) hai cua tre".
• Adding a wildcard symbol may retrieve word variants: thu sach ky nang ((x ho) or (o hoi)). hieu qua cac duc ((va dau) OR (va dao)) hai cua tre*.