Perhaps you should try some spelling variations:
vien cac » vien bach, vien dai, vien da
doan thi » dan thi, doanh tai, hoang thi
sach ty » sach y, sach tu, sach thu
tu sach » thu sach, su sach, te sach
lang xa » lang ha, dang xay, lang van
ty lang » tay lang, ty xang, ty hang
cua tre » cua te, cuu tre, cua triet
ve cac » va cac
toi va » thoi va, tuoi va, hoi va
dan vi » doan vi, don vi, dan va

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 ty lang xa hoi. hieu ((ve cac) OR (vien cac)) toi va ((dan vi) or (doan thi)) cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: tu sach ty lang xa hoi. hieu ((ve cac) OR (vien cac)) toi va ((dan vi) OR (doan thi)) cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "tu sach ty lang xa hoi. hieu ((ve cac) OR (vien cac)) toi va ((dan vi) or (doan thi)) cua tre".
• Adding a wildcard symbol may retrieve word variants: tu sach ty lang xa hoi. hieu ((ve cac) OR (vien cac)) toi va ((dan vi) or (doan thi)) cua tre*.