Perhaps you should try some spelling variations:
hieu van » hieu va, thien van, lieu va
su sach » so sach, vu sach
sach ky » sac ky, sach y, sach ty
van cau » van dau, van hau, van chat
cau hoc » cau hoi, cau hoa, cao hoc
hanh va » hanh vi
cua tre » cua te, cuu tre, cua triet
x hoi » o hoi, x ho, 5 hoi

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

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