Perhaps you should try some spelling variations:
sang mai » sang ma, sang tai, lang mai
dung doi » huong doi, dung co, dung mo
su sach » so sach, vu sach
hieu va » lieu va, hieu qua
cac tuc » cac toc, cac tu, cac tac
cua tre » cua te, cuu tre, cua triet
sach ky » sac ky, sach y, sach ty
hieu ve » hieu van
mai hoi » mai khoi, dai hoi, lai hoi
ky sang » ky nang, ky dang, ky tang

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 ky sang mai hoi. ((hieu ve) or (hieu va)) cac tuc xa ((dung doi) OR (dung tai)) cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: su sach ky sang mai hoi. ((hieu ve) OR (hieu va)) cac tuc xa ((dung doi) OR (dung tai)) cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "su sach ky sang mai hoi. ((hieu ve) or (hieu va)) cac tuc xa ((dung doi) OR (dung tai)) cua tre".
• Adding a wildcard symbol may retrieve word variants: su sach ky sang mai hoi. ((hieu ve) or (hieu va)) cac tuc xa ((dung doi) OR (dung tai)) cua tre*.