Perhaps you should try some spelling variations:
hieu vien » hieu viec, thieu nien, hieu tieng
hieu hien » bieu hien, dieu hien, chieu chien
cau tren » cu tren, dau tren, ca tren
bac cao » bao cao, bac can, bac cau
sach ky » sac ky, sach y, sach ty
tu sach » thu sach, su sach, te sach
bac ca » bac can, bac cau, bac ha
dan vi » doan vi, don vi, dan va
vi cau » va cau, ve cau, di cau
x hoi » xa hoi, o hoi, x ho

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. ((hieu vien) or (hieu hien)) cac ((bac ca) OR (bac cao)) dan vi cau tren~.
• If you are trying to use Boolean operators, they must be ALL CAPS: tu sach ky nang x hoi. ((hieu vien) OR (hieu hien)) cac ((bac ca) OR (bac cao)) dan vi cau tren.
• The words AND, OR and NOT may confuse the search; try adding quotes: "tu sach ky nang x hoi. ((hieu vien) or (hieu hien)) cac ((bac ca) OR (bac cao)) dan vi cau tren".
• Adding a wildcard symbol may retrieve word variants: tu sach ky nang x hoi. ((hieu vien) or (hieu hien)) cac ((bac ca) OR (bac cao)) dan vi cau tren*.