Perhaps you should try some spelling variations:
hieu van » hieu va, thien van, lieu va
sach ty » sach tu, sach thu, xach tay
van bac » van ban, van bao, van hac
cua tre » cua te, cuu tre, cua triet
nang o » nang co, nang ho, nang ro
ho van » hoc van, so van, do van
vi cua » vai cua, di cua
o hoi » co hoi, o hoc, o hai

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

• Performing a fuzzy search may retrieve terms with similar spellings: toc ((sach ty) OR (sach y)) nang o hoi. hieu van bac ((ho van) or (vo van)) hanh vi cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: toc ((sach ty) OR (sach y)) nang o hoi. hieu van bac ((ho van) OR (vo van)) hanh vi cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "toc ((sach ty) OR (sach y)) nang o hoi. hieu van bac ((ho van) or (vo van)) hanh vi cua tre".
• Adding a wildcard symbol may retrieve word variants: toc ((sach ty) OR (sach y)) nang o hoi. hieu van bac ((ho van) or (vo van)) hanh vi cua tre*.