Can I add video which were made oleh somebody else!For example I downloaded video from Youtube, will it be legal to add it here?

well its okay because fanpop doesnt actually HOST the video they are just linked. anda dont download it to your computer and upload it here (which would be sketchy if anda didnt make it).
anda post them as a link and any video can be traced to the original hosting site (youtube for example) oleh clicking the little blue panah selanjutnya to the judul (see image).
thats why- if i really like something- i always click on that link and komentar and rate the original as well as the fanpop content but since not every one does that i think its good to credit the video maker.
p.s. if the creator doesnt want their video to be diposting elsewhere they will disable embedding so anda can only post it as a link and not a video thus bringing the viewer to the original content.

yes anda can, but anda have to mention who made the video, atau else everybody thinks anda made it ;)