Hi, Debbie,
Here’s a You Tube iMovie tutorial that will be helpful:
https://www.youtube.com/watch?v=ljAWrpmEFoU
Here’s a good online help menu that explains how to use the features of iMovie 10:
https://help.apple.com/imovie/mac/10.1/
You can play your project by clicking on the triangle below the preview screen. It should play smoothly from the place where you have placed the playhead without jumping to the end. If it jumps to the end, it might be that you just need to reset (delete) preferences, which is a very safe procedure. To delete preferences, openiMovie 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. Now see if the jumping issue is cured.
You can also try creating a new project and see if it plays properly.
-- Rich