How do I draw straight lines?

[Gerhard Huber]

Use a vector line or the drawing tools.
All drawing tools (painting, rubber, stamp, finger, ...) are able to draw
lines. Simply hold Ctrl + Alt (Windows) or Command + Alt (Mac OS) and drag a line.

[Gerhard Huber]

from version 16 on the modifier to create lines will be Shift.

Gerhard