How to Add Clips in Windows Movie Maker

You build a movie by adding clips to it from your media library.

Drag a clip to the timeline

In the Libraries list, select the video clip you want to add to your movie, drag a clip from the browser to the timeline. It can be the main video track or the PIP video track. If it's the main video track, the default position must be the beginning position or behind other clips. The video clip at the PIP track can be any position.

Replace a clip in the timeline

Right-click your selection on top of the clip you want to replace in the timeline, select "Change Media".

It will pop up the folder with your computer, then select image files, or video files to target with it, and click it to replace with it.  The replaced clip will keep the original clip’s duration and using the new clip’s start point as the beginning.

Tips: 

If you add 4:3 standard-definition video to a 16:9 high-definition project,  it will automatically crop the video to fit the full 16:9 frame. To show the full 4:3 frame, you can set a new project with 4:3.