Perhaps you should try some spelling variations:
van khanh » van thanh, van hanh, van anh
van nhanh » van thanh, van hanh, van nhan
cau tren » cu tren, dau tren, ca tren
thai cau » thai cao, toan cau, thai van
cac duc » cac du, cao duc, cac dac
tu sach » thu sach, su sach, te sach
sach ky » sac ky, sach y, sach ty
hieu ve » hieu va, hieu van
ve cac » va cac, vien cac
o hai » co hai, so hai, o lai
o hoi » co hoi, o hoc, o hoa

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

• If you are trying to use Boolean operators, they must be ALL CAPS: tu sach ky nang ((o hoi) OR (o hai)). hieu ve cac duc ((van nhanh) OR (van khanh)) thai cau tren~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "tu sach ky nang ((o hoi) or (o hai)). hieu ve cac duc ((van nhanh) OR (van khanh)) thai cau tren~".
• Adding a wildcard symbol may retrieve word variants: tu sach ky nang ((o hoi) or (o hai)). hieu ve cac duc ((van nhanh) OR (van khanh)) thai cau tren*.