Perhaps you should try some spelling variations:
tu sach » thu sach, su sach, te sach
sach ky » sac ky, sach y, sach ty
hieu ve » hieu va, hieu van
cua tre » cua te, cuu tre, cua triet
va dan » ca dan, va dao, va dau
cac tu » cac thu, cach tu, tac tu
dan vi » doan vi, don vi, dan va
vi cua » vai cua, di cua
o hoi » co hoi, o hoc, o hai
x hoi » xa 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: tu sach ky nang ((x hoi) OR (o hoi)). hieu ve ((cac tu) or (cac du)) va dan vi cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: tu sach ky nang ((x hoi) OR (o hoi)). hieu ve ((cac tu) OR (cac du)) va dan vi cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "tu sach ky nang ((x hoi) OR (o hoi)). hieu ve ((cac tu) or (cac du)) va dan vi cua tre".
• Adding a wildcard symbol may retrieve word variants: tu sach ky nang ((x hoi) OR (o hoi)). hieu ve ((cac tu) or (cac du)) va dan vi cua tre*.