Whit Brachna's "Masks Demonstration" video
download costs a mere $5 from this very site.
My 'doh' moment when I realised how great masking was came with reading a
tutorial that involved adjustment layers and masking. Up until that point I just didn't get it either.
Masks are great because they are non-destructive. You can remove parts of a layer fully or partially and just as easily bring them back... something you can't do with the eraser or otherwise deleting pixel information.
Your second problem is harder. As long as you keep pressing down and continuing the one stroke you can go back over lines and areas... as soon as you lift the pen at all you will get the behaviour you describe. If you want a an area to be (for example) 50% grey, then don't use black with less pressure - use 50% grey at high pressure. You can't expect the Brush tool in Photoshop to read your mind.
What you are seeing as an unwanted behaviour is something that is key to the way many people use Photoshop for digital art. They allow tone and colour to build up successively in stages, ALT click to sample intermediate tones and colours that are created by building up strokes and do a bit of smoothing that way.
I always found I preferred both opacity and flow to be controlled by pressure, while leaving Opacity set at 100% (allowing pen to control fully) and capping Flow at a lower figure around 15% (pen pressure controls flow but it can't be higher than 15)
I do however far prefer Painter 11 these days.
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