Perhaps you should try some spelling variations:
hieu quan » lieu quan, thien quan, lien quan
cua viet » ca viet, cua vat, cua dieu
nang mai » nang ma, nang tai, lang mai
nam thuc » nam thuoc, am thuc, nam thu
tu sach » thu sach, su sach, te sach
ho nang » thi nang, kha nang, cao nang
mai hoi » mai khoi, dai hoi, lai hoi
sach ho » sach hoi, sach ha, sach lo
thuc va » thuc vat, thuc dan, thuc day
va hanh » van hanh, va thanh, va danh

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 ho nang mai hoi. ((hieu quan) or (hieu qua)) nam thuc va hanh vi ((cua viet) OR (hoa viet))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: tu sach ho nang mai hoi. ((hieu quan) OR (hieu qua)) nam thuc va hanh vi ((cua viet) OR (hoa viet)).
• The words AND, OR and NOT may confuse the search; try adding quotes: "tu sach ho nang mai hoi. ((hieu quan) or (hieu qua)) nam thuc va hanh vi ((cua viet) OR (hoa viet))".
• Adding a wildcard symbol may retrieve word variants: tu sach ho nang mai hoi. ((hieu quan) or (hieu qua)) nam thuc va hanh vi ((cua viet) OR (hoa viet))*.