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i personally prefer to use S-4 for default drawing and S-7 when i need to draw really long smooth lines. this is what it does:

this is where you find transparent brush:

when you enable it, you can use your currently selected brush as eraser. this is what it does:

this is where you find clipping group:

you have to have at least two layers, the base layer and the one for the colors. when you draw on the layer with little red bar on side, the stuff you put on the layer appears only on top of the things you have on the base layer. the effect is p much same as preserving opacity except you dont change anything about the base layer and you can still use layer textures and effects. this is what it does: