Perhaps you should try some spelling variations:
doc sach » toc sach, nuoc sach, dac sac
hanh tai » thanh tai, hang tai, hinh tai
nuoc van » nuoc va, nuoc san, luoc van
cac nuoc » cap nuoc, hoc nuoc, dat nuoc
hoc sach » hoc sinh, hoc cac, hoc anh
qua cac » cua cac
han tai » hang tai, nhan tai, van tai
cua tre » cua te, cuu tre, cua triet
x hoi » xa hoi, o hoi, x ho

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

• Performing a fuzzy search may retrieve terms with similar spellings: ((doc sach) or (hoc sach)) ky nang x hoi. hieu qua cac nuoc van ((hanh tai) OR (han tai)) cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: ((doc sach) OR (hoc sach)) ky nang x hoi. hieu qua cac nuoc van ((hanh tai) OR (han tai)) cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((doc sach) or (hoc sach)) ky nang x hoi. hieu qua cac nuoc van ((hanh tai) OR (han tai)) cua tre".
• Adding a wildcard symbol may retrieve word variants: ((doc sach) or (hoc sach)) ky nang x hoi. hieu qua cac nuoc van ((hanh tai) OR (han tai)) cua tre*.