Perhaps you should try some spelling variations:
pham thuc » phap thuc, pham chuc, nham thuc
thu sach » thuat sach, thuoc sach, thu hoach
hieu van » thien van, lieu va, hieu ve
tu sach » su sach, te sach, tu cach
cao hoi » cau hoi, co hoi, cao hoc
hieu va » lieu va, hieu qua
thuc va » thuc vat, thuc dan, thuc day
dan vai » dan va, dan tai, dan vat
va dan » ca dan, va dao, va dau
vai va » mai va, dai va, hai va
thiet » thieu

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

• Performing a fuzzy search may retrieve terms with similar spellings: ((tu sach) or (thu sach)) ky nang cao hoi. ((hieu van) OR (hieu va)) pham thuc va dan vai va thiet~.
• If you are trying to use Boolean operators, they must be ALL CAPS: ((tu sach) OR (thu sach)) ky nang cao hoi. ((hieu van) OR (hieu va)) pham thuc va dan vai va thiet.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((tu sach) or (thu sach)) ky nang cao hoi. ((hieu van) OR (hieu va)) pham thuc va dan vai va thiet".
• Adding a wildcard symbol may retrieve word variants: ((tu sach) or (thu sach)) ky nang cao hoi. ((hieu van) OR (hieu va)) pham thuc va dan vai va thiet*.