Perhaps you should try some spelling variations:
toc sach » hoc sach, doc sach, thuoc sach
van hanh » van thanh, ban hanh, van anh
cac nuoc » cap nuoc, dat nuoc, cua nuoc
hieu ve » hieu va, hieu van
nang ma » tang ma, sang ma, nang cao
lang ha » long ha
cua tre » cua te, cuu tre, cua triet
vi cua » vai cua, di cua

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 y ((nang ma) or (lang ha)) hoi. hieu ve ((cac nuoc) OR (hoc nuoc)) van hanh vi cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: toc sach y ((nang ma) OR (lang ha)) hoi. hieu ve ((cac nuoc) OR (hoc nuoc)) van hanh vi cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "toc sach y ((nang ma) or (lang ha)) hoi. hieu ve ((cac nuoc) OR (hoc nuoc)) van hanh vi cua tre".
• Adding a wildcard symbol may retrieve word variants: toc sach y ((nang ma) or (lang ha)) hoi. hieu ve ((cac nuoc) OR (hoc nuoc)) van hanh vi cua tre*.