Perhaps you should try some spelling variations:
doc sach » toc sach, nuoc sach, dac sac
hoc sach » hoc sinh, hoc cac, hoc anh
vai cuu » vai cua
dan vai » dan va, dan tai, dan vat
thuc va » thuc vat, thuc dan, thuc day
cuu tre » cua tre, cau tren, cau treo
tang xa » trang xa, tang ma, lang xa
ky tang » ky nang, ky dang, kho tang
va nay » va ngay, va cay
va dan » ca dan, va dao, va dau
va nam » van nam, ha nam

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

• Performing a fuzzy search may retrieve terms with similar spellings: ((doc sach) OR (hoc sach)) ky tang xa hoi. hieu ((va nam) or (va nay)) thuc va dan vai cuu tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: ((doc sach) OR (hoc sach)) ky tang xa hoi. hieu ((va nam) OR (va nay)) thuc va dan vai cuu tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((doc sach) OR (hoc sach)) ky tang xa hoi. hieu ((va nam) or (va nay)) thuc va dan vai cuu tre".
• Adding a wildcard symbol may retrieve word variants: ((doc sach) OR (hoc sach)) ky tang xa hoi. hieu ((va nam) or (va nay)) thuc va dan vai cuu tre*.