I'm no expert, but here are a couple ideas for your "Tv" and "Av" assignment. it sounds like the purpose of your assignment is to show how using either "Tv" or "Av", you can take better pictures than the camera on "Auto".
For "shutter priority" or "Tv", you the photographer control the shutter speed. So you have to find situations where controlling the shutter speed is important for your picture. Most commonly, you would use "shutter priority" in situations where there is a fast moving object that you want to take a picture of. For example, a sprinter on a race track, a car driving by, a squirrel rushing past you, a child riding a bicycle, taking pictures on a moving boat, etc. In any of these above situations, the camera doesn't know that you're taking pictures of a fast-moving object, so when you take pictures of these situations using "Auto", you will most certainly get blurry pictures.
Only you, the photographer, know that the subject in front of you is a fast-moving object. This is a situation where you would set the camera to "shutter priority" or "Tv" and set the camera to a fast shutter speed. Using a fast shutter speed will "freeze" the motion in your photo.
Or how about another example: have your husband drive you on the freeway while you're in the passenger seat taking pictures with your camera. At 60 mph, any pictures of nearby scenery that you take on "Auto" will be blurry. However, if you set your camera to "Tv" and use a shutter speed of, say, 1/1000 sec, you should be able to get sharp, non-blurry pictures.
Or, for a bonus, there might be situations where you'd actually want a SLOWER shutter speed than what the camera selects. First of all, to use slower shutter speeds, you'd probably want to use a tripod. Are you near a beach, lake, or river? If you take a picture of any of these on "Auto", you'll get a nice, sharp, standard picture of a body of water. However, if you set your camera on "Tv" and use a SLOWER shutter speed, you can make the body of water appear silky smooth.
Another situation to use slower shutter speed is if you want to take pictures of streaks of light, such as cars along the freeway at night, a spinning merry-go-round, a spinning wheel, etc. If you do it right, using slower shutter speeds can actually convey a sense of motion, such as the motion of water, the motion of the merry-go-round, the motion of cars, the motion of spinning wheels, etc.
As mentioned earlier, you would use "aperture priority" or "Av" to select an aperture that's different from what the camera would select on "Auto". As you may recall from your photography classes, changing the aperture actually changes the depth of field in your photo. When you use a large aperture (small F-number, such as f/2.8), your subject will be in focus, but your background will be beautifully blurry. On the other hand, when you use a small aperture (large F-number, such as F/16 or F/22), this not only will your subject be in-focus, but your background will also be in-focus as well.
There are a number of different situations were you can take pictures to demonstrate different apertures and, thus, different depths of field. Can you line up 3 to 5 bottles (or toys, soda cans, etc) on a bright countertop or table? Get within about 1-2 feet from the closest bottle, and set your camera to focus on the bottle that's closest to you. If you use a large aperture (small f-number), you'll see that the more distant bottles get more and more blurry. On the other hand, if you use a small aperture (large f-number), all the bottles will appear like they're in focus. I would guess that on "Auto", the camera would probably get 1/2 the bottles in-focus.
Here's an example from someone else's website of what I'm talking about:
If you're out in the park, try to find a bunch of ducks, trees, etc., where there is at least one subject that is closest to you and other subjects that are far far away from you. For the duck example, get down to "duck level", and focus on the duck that is closest to you. When you use a large aperture (small f-number), all the distant ducks will be blurry and any other scenery in the distant background will also be blurry. However, when you use a small aperture (large f-number), all the distant ducks will be in-focus and any other scenery in the distant background will also be in-focus.
In the end, you'll find that the purpose of this assignment will show you that you, the photographer, are smarter than the camera on "Auto". The camera doesn't know that you're taking pictures of fast-moving objects, and the camera doesn't know that you want to blur the background to make your subject "pop" out.
Hope that helps.
