Perhaps you should try some spelling variations:
van hanh » van thanh, ban hanh, van anh
hanh tai » thanh tai, hang tai, han tai
sach ty » sach tu, sach thu, xach tay
hieu ve » hieu va, hieu van
cac duc » cac du, cao duc, cac dac
duc van » duc va, duc tan, duc manh
tai cua » thai cua, tai cuc, tri cua
cua tre » cua te, cuu tre, cua triet
nang o » nang co, nang ho, nang ro
ve cac » va cac, vien cac
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: te ((sach ty) or (sach y)) nang o hoi. hieu ve cac duc van hanh tai cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: te ((sach ty) OR (sach y)) nang o hoi. hieu ve cac duc van hanh tai cua tre*.