Use Line/P,P to create a line through two existing points, or Line/X,Y,X,Y to create a line from explicit coordinates.
Multiple ways to create lines: Line/P,P (through two points), Line/X,Y,X,Y (explicit coordinates), Line/Parlel,XAxis,D (horizontal), Line/Parlel,YAxis,D (vertical), Line/P,Atangl (at angle).