Perhaps you should try some spelling variations:
trinh may » trinh ky, trinh han, trinh ly
doc sach » hoc sach, toc sach, nuoc sach
tinh may » tinh ma, tinh tay, tinh mach
te sach » toc sach
cao duc » dao duc, cao suc, giao duc
cua tre » cua te, cuu tre, cua triet
hieu ve » hieu va, hieu van
tang ma » mang may, tang van
ma hoi » mai hoi, xa hoi, ma hoa
y tang » ky tang, 6 tang, 7 tang
ve cao » va cao, ve bao, ve cac

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) or (doc sach)) y tang ma hoi. hieu ve cao duc va ((tinh may) OR (trinh may)) cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: ((te sach) OR (doc sach)) y tang ma hoi. hieu ve cao duc va ((tinh may) OR (trinh may)) cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((te sach) or (doc sach)) y tang ma hoi. hieu ve cao duc va ((tinh may) OR (trinh may)) cua tre".
• Adding a wildcard symbol may retrieve word variants: ((te sach) or (doc sach)) y tang ma hoi. hieu ve cao duc va ((tinh may) OR (trinh may)) cua tre*.