in iMovie my clips wont playback, it just shoots to the end

Try deleting preferences and see if that cures it. To delete preferences, open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Possibly you may need to reboot to set the changes.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

You can also try placing your media into a newly created project and see if iMovie functions correctly there.

-- Rich