Perhaps you should try some spelling variations:
"su sach bai nghe duoc hoc" » "su sach bai nghiem duoc hoc", "su sach bai nghe nuoi hoc", "su sach bai che duoc hoc", "su sach dao nghe duoc hoc", "su cac bai nghe duoc hoc", "su sach xa nghe duoc hoc", "su sach thai nghe duoc hoc", "su sach van nghe duoc hoc", "su sach nam nghe duoc hoc", "su sach bai nghe quoc hoi", "so sach bai nghe duoc hoc", "vu sach bai nghe duoc hoc"
"tu cach ha nghe tu hoc" » "tu cac ha nghe tu hoc", "tu cach pha nghe tu hoc", "tu bach ha nghe tu hoc", "tu sach ha nghe tu hoc", "tu cach hoa nghe tu hoc", "tu cach va nghe tu hoc", "tu cach da nghe tu hoc", "tu cach ha nghe thu hoc", "tu cach ha nghe xu hoc", "tu cach ha nghi tu hoc", "tu cach ha nghe te hoc"

Try widening your search to All Fields.

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 bai nghe duoc hoc") or ("tu cach ha nghe tu hoc"))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("su sach bai nghe duoc hoc") OR ("tu cach ha nghe tu hoc")).
• Removing quotes may allow a broader search: (( su sach bai nghe duoc hoc ) or ( tu cach ha nghe tu hoc )).
• Adding a wildcard symbol may retrieve word variants: (("su sach bai nghe duoc hoc") or ("tu cach ha nghe tu hoc"))*.