I have a polygon which I want to split with a selected line with a parallel line.
I have made the parallel line but it's lenght is not enought to split the polygon.
It necessary to be parallel with the other line. Can I change the line lenght without I change the position of the line?
If you are trying to extend your parallel line while keeping its orientation (angle), please try following steps.
Before this, you need to make sure "Advanced Digitizing Panel" is displayed by View | Panels |(check) Advanced digitizing panel.
You can also set the angle to a fixed number, and lock, in the "a" entry of the Advanced Digitizing Panel. (I do not know which way is easier for your case.)
To do this easily, setting Snapping Option is highly recommended.