Perhaps you should try some spelling variations:
dua tren » dung tren
lang mai » lang dai, lang hai, lang man
dat duoc » dat nuoc, da duoc, luat duoc
tu sach » thu sach, su sach, te sach
va hanh » van hanh, va thanh, va danh
duoc va » nuoc va, duc va, duoc ma
hieu ve » hieu va, hieu van
sach ky » sac ky, sach y, sach ty
nang ma » tang ma, sang ma, nang cao
cua tre » cua te, cuu tre, cua triet
ve dat » de dat, va dat, ve dao

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 ky ((nang ma) or (lang mai)) hoi. hieu ve dat duoc va hanh vi ((dua tren) or (cua tre))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: tu sach ky ((nang ma) OR (lang mai)) hoi. hieu ve dat duoc va hanh vi ((dua tren) OR (cua tre)).
• Adding a wildcard symbol may retrieve word variants: tu sach ky ((nang ma) or (lang mai)) hoi. hieu ve dat duoc va hanh vi ((dua tren) or (cua tre))*.