Perhaps you should try some spelling variations:
toc sach » hoc sach, doc sach, thuoc sach
van hanh » van thanh, ban hanh, van anh
nhan luc » nang luc
chat luc » chat xuc, chat lua, chat huu
hieu ve » hieu va, hieu van
van hoi » van hoa, van hoc, van hoe
cua tre » cua te, cuu tre, cua triet
y tang » ky tang, 6 tang, 5 tang
7 tang » 6 tang
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 tang) or (7 tang)) van hoi. hieu ve ((chat luc) OR (nhan luc)) van hanh vi cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: toc sach ((y tang) OR (7 tang)) van hoi. hieu ve ((chat luc) OR (nhan luc)) van hanh vi cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "toc sach ((y tang) or (7 tang)) van hoi. hieu ve ((chat luc) OR (nhan luc)) van hanh vi cua tre".
• Adding a wildcard symbol may retrieve word variants: toc sach ((y tang) or (7 tang)) van hoi. hieu ve ((chat luc) OR (nhan luc)) van hanh vi cua tre*.