Perhaps you should try some spelling variations:
vien cac » vien bach, vien dai, vien da
su sach » so sach, vu sach
sach ky » sac ky, sach y, sach ty
nang ro » nang co, nang ra, nang rong
nang ho » nang hoc, nang co, tang ho
cua tre » cua te, cuu tre, cua triet
ve cac » va cac
va don » va dong, va doc, va do
vi cua » vai cua, di cua

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

• Performing a fuzzy search may retrieve terms with similar spellings: su sach ky ((nang ho) or (nang ro)) hoi. hieu ((ve cac) OR (vien cac)) tu va don vi cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: su sach ky ((nang ho) OR (nang ro)) hoi. hieu ((ve cac) OR (vien cac)) tu va don vi cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "su sach ky ((nang ho) or (nang ro)) hoi. hieu ((ve cac) OR (vien cac)) tu va don vi cua tre".
• Adding a wildcard symbol may retrieve word variants: su sach ky ((nang ho) or (nang ro)) hoi. hieu ((ve cac) OR (vien cac)) tu va don vi cua tre*.