Shape tweens are an easy way of changing shape of an object on the stage.
First, place an object such as a circle or square on the stage. The first frame should turn gray to show that there is an object on the stage. Then, right-click a frame later in the timeline and select 'insert keyframe', or just hit F6. The frames between the first frame and the keyframe should turn gray.
Click on the second keyframe to select it, and use the selection tool (the black arrow), or the free transform tool (the white box with a black border and small black squares on the side) to change the shape of your object on the stage.
To create the shape tween, select a frame between the two keyframes. In the properties panel, there is a drop-down menu next to the word 'tween'. Select 'shape' from the drop-down menu, and the frames between the two keyframes turn solid green with a long, solid line ending in an arrow.
To preview your animation, press Ctrl+Enter, or hit F12 for a more accurate HTML preview of what your animation will look like.
Note that you cannot tween an object unless it is a vector. To turn a non-vector object into a vector, use the Modify > Break Apart command.
You can use "hints" to control your tween. To add a hint, go to Modify > Shape > Add Shape Hint.