Perhaps you should try some spelling variations:
sach ky » sach y, sach ty, sach kem
qua cac » cua cac
cac duc » cac du, cao duc, cac dac
cau hai » cau hoi, cau hat, cau ha
hai cua » thai cua, dai cua, hoi cua
cua tre » cua te, cuu tre, cua triet
sac ky » cac ky, sai ky
va cau » van cau, va cac, va dau
x hoi » xa hoi, x ho, 5 hoi
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) OR (sac ky)) nang ((x hoi) or (o hoi)). hieu qua cac duc va cau hai cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: thu ((sach ky) OR (sac ky)) nang ((x hoi) OR (o hoi)). hieu qua cac duc va cau hai cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "thu ((sach ky) OR (sac ky)) nang ((x hoi) or (o hoi)). hieu qua cac duc va cau hai cua tre".
• Adding a wildcard symbol may retrieve word variants: thu ((sach ky) OR (sac ky)) nang ((x hoi) or (o hoi)). hieu qua cac duc va cau hai cua tre*.