To animate a mask over time for a shot with changing camera positions, follow these steps:

Setup the Initial Mask: 

Set the mask over the part of the video you want to isolate. This will be the first key control frame point, shown in the left-hand monitor. This initial position of the mask will be the starting point for the animation.

Unlock Control Keyframe: 

Click on the padlock button to unlock the control keyframe. This will give you independent control over the mask's size and position from the beginning of the video clip to the end.

Set the End Keyframe:

Select the mask on the end keyframe control point, which is not covering the face in the video. Now, drag the mask's position to cover the face on the right monitor in the Video Effect window.

Animate the Mask: 

By dragging the mask's position between the first and end keyframes, you are animating the mask over time. The animation bar in the Video Effect Control Interface will show the mask's motion over time, measured in percentage.

Preview the Animation: 

Play the video to see the results of your animated mask. The mask should now follow the changing position of the face or object in the video.

Close the Subtrack:

After you are satisfied with the animation, close the subtrack by right-clicking on the track selection. This will finalize the blur effect animation.

In the next tutorial, you will learn how to handle shots with uneven motion that may require more than two keyframes for a smoother animation. Additionally, you will learn how to fade the blur effect out for a seamless transition.

By mastering the techniques of animating masks over time, you can effectively blur objects or faces in videos that have changing camera positions or movements.