Perhaps you should try some spelling variations:
tu sach » thu sach, su sach, te sach
hieu ve » hieu va, hieu van
cua tre » cua te, cuu tre, cua triet
y nang » ky nang, ly nang, v nang
cac tu » cac thu, cach tu, tac tu
va dan » ca dan, va dao, va dau
dan vi » doan vi, don vi, dan va
vi cua » vai cua, di cua
o hoc » co hoc, mo hoc, so hoc
o hoi » co hoi, o hai, o hoa

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 y nang ((o hoi) OR (o hoc)). 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 y nang ((o hoi) OR (o hoc)). 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 y nang ((o hoi) OR (o hoc)). hieu ve ((cac tu) or (cac du)) va dan vi cua tre".
• Adding a wildcard symbol may retrieve word variants: tu sach y nang ((o hoi) OR (o hoc)). hieu ve ((cac tu) or (cac du)) va dan vi cua tre*.