How can most correctly to draw in toon boom? By the brush or by the polyline? And how can most correctly to use the polyline?
Accuracy will depend on the situation, your skill and comfort with the tools. Sometimes a polyline will be best. I almost never use it. Sometimes drawing freehand will be easiest to achieve accuracy but you may prefer to use other options.
For instance, I am comfortable freehand drawing with the tools in Photoshop, Illustrator and on traditional paper with real pencils and pens. I still prefer traditional pen/pencil and paper. I like drawing with Photoshop and Illustrator. I am not as comfortable with using the drawing tools in Toon Boom.
Specifically, I find that there is a degree of resistance in PS, Illustrator and traditional tools that is missing at least in the default settings within TB. I have to dig deeper to see if I can make adjustments. As it is by default the tablet tip slides too easily and does not stop the way I would like. I end up with little squiggly points at the ends of lines like you would get when squirting whipping cream on an ice cream sundae or yogurt into a cup.
I do not really feel comfortable using a tablet but it is easier to me using PS and Illustrator. However, I draw on traditional paper, scan then import the roughs and use them as guides while I take extra care drawing the final linework freehand within TB. To me it is faster to draw freehand and then utilize the control points in a vector line than to bother with the polyline tool. Additionally, if I am going to do it in this many steps I prefer retaining the beneficial (to me) tactile character of traditional paper and pencil/pens.
As for choosing between the Brush or Pencil, IMO the Pencil offers the most benefit because it is the most flexible once you have completed the drawing. In Animate Pro and Harmony there are a lot of interesting pencil styles and you can manipulate the lines with unique tools.
Pencil Tool 13:25