First of all, if you are in manual (M), I assume you also have an internal exposure meter with some indicator that tells you that the picture is at the correct exposure, that is per aperture, shutter speed based on your iso.
Depth of field and focus touch on a photograper's idea of composition and what he feels is important or what he would like others to SEE in his shot. One uses the aperture to do that. You might think of your eyes and eyelids as a natural aperture.
HINT: If you're squinting, are you letting more light into your eyes than before and by squinting, do objects become more in focus or not?
So, let's say in shot number 3, you wanted to stress the importance of that shock absorber(?) there in the front. I would first zoom in on the SA until I had it in focus. Second, open up the aperture until you get the desired amount of blur in the background leaving the SA in focus. Now, I am assuming that your camera has a depth of field preview which will allow you to see this. No matter.
Then, adust the shutter speed until your exposure meter says the pic is properly exposed. Let' say that orginally, at proper exposure, the shutter was around f/8 and you took it to f/2. That means you let in more light. That means you have to speed up the shutter to compensate for the "excess" light. Therefore, if the shutter speed was at say 1/125 of a second, you may have to take it to 1/250 or 1/500 of a second to be at the right exposure. Want everything in focus, do the reverse. Make sense?
Rule of thumb. if you want to blur the background, use apertures going toward the small numbers like f/4,2,1.8 etc. If you want everything in focus, then go towards the larger numbers f/16, 32 etc. Then set your shutter appropriately.
hope that helps.
