Perhaps you should try some spelling variations:
doc sach » hoc sach, toc sach, nuoc sach
nam thuc » nam thuoc, am thuc, nam thu
cau tren » cu tren, dau tren, ca tren
tang ma » mang may, tang van
tang xa » trang xa, lang xa, tang ha
sach ky » sac ky, sach y, sach ty
vai cau » va cau, van cau, vai cac
hieu va » lieu va, hieu qua
va nam » van nam, ha nam, va nay
ca dan » ca dao

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 ky ((tang xa) or (tang ma)) hoi. hieu va nam thuc ((ca dan) OR (cac dan)) vai cau tren~.
• If you are trying to use Boolean operators, they must be ALL CAPS: doc sach ky ((tang xa) OR (tang ma)) hoi. hieu va nam thuc ((ca dan) OR (cac dan)) vai cau tren.
• The words AND, OR and NOT may confuse the search; try adding quotes: "doc sach ky ((tang xa) or (tang ma)) hoi. hieu va nam thuc ((ca dan) OR (cac dan)) vai cau tren".
• Adding a wildcard symbol may retrieve word variants: doc sach ky ((tang xa) or (tang ma)) hoi. hieu va nam thuc ((ca dan) OR (cac dan)) vai cau tren*.