when i am trying to download any video, to see if its the right one i try to preview the file but nothing happens.i click on the preview button,double click it,and right click and click on preview. probably pretty easy to solve but im stupid.

To preview a file you're downlding it must have downlded a certain % before you can preview it. For example, a song that's 4 mins long & you've downlded 5%, then you'll only be able to preview about 12 secs.

To preview, select the file & press preview button. But it needs to be more than 0% downlded. You can't preview what you haven't yet downlded.

i am downloading a VIDEO. it on 33% and the size of it is 170.8 mb and it still wont

Did you try previewing from the search results or downld window. Choose downld window. Keep in mind very large video files might take some time to load & open.

Try this, get VLC: http://www.videolan.org/vlc/ (choose nearest mirror site), & install it. Make it your default video program. Then try previewing the file. To find out how to make it your default see Setting Default video app

If later you'd prefer VLC not to be your default video player, that's fine, just revert the defaults back as described.