Perhaps you should try some spelling variations:
nghien su » nghiem su, nghien cuu, nghiep sua
sach dat » sach dao, sach danh, sach bai
dat nghe » day nghe, da nghe, dang nghe
hoach va » hoach xay, hanh va, doanh va
tu sach » thu sach, su sach, te sach
nghe d » nghe di, nghe _, nghe 新
su hoc » tu hoc, du hoc, so hoc
d hoc » du hoc, ad hoc, de hoc

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 dat nghe d hoc ) or ( thu hoach va nghien su hoc ))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (( tu sach dat nghe d hoc ) OR ( thu hoach va nghien su hoc )).
• Adding a wildcard symbol may retrieve word variants: (( tu sach dat nghe d hoc ) or ( thu hoach va nghien su hoc ))*.