The Line tool is used in much the same way that you would use a real pencil to draw with a ruler. Simply select the tool and drag on the Stage to draw a straight line between the start and end points.
SHIFT key : Constrain angles
You can press the Shift key while dragging to constrain possible angles for the line. By default this will only allow for lines that are either horizontal, vertical or 45 degrees between.
You can specify the color with the Stroke Color Tool.
You can turn a straight line into a curve with the Arrow tool
1: Make sure the line is not selected (you can do this by clicking somewhere on an empty spot on the stage).
2: Select the Arrow tool.
3: Click on the line, and while holding down the mouse button, drag the curve.
4: Release the mouse button and your curve is finished.
Curving Lines and Edges:
You can move a line with the mouse, quite similar to how you'd turn it into a curve or you can move it with the arrow keys.
1: Select the Arrow tool.
2: Make sure the line is selected (you can do this by clicking once on the line then release the mouse button).
3a: Click on the line, and while holding down the mouse button, move it to the new position.
3b: Press the arrow keys on your keyboard to move the line.
Precise size and positioning
You can position the line perfectly by using the info panel.
Info Panel : Window > Panels > Info
The "square" connected to the line where it says "Click this square" is used to set the origin for coordinates - either at top left or center of the stage.
Fields for X and Y are used to enter the position of the selected object.
Fields for W and H are used to enter the width and height of the selected object.
When working with lines, one might think that height would be 1 for a thin line. However, width and height refers to the square between the start and end points of the selected object.
A horizontal line that is 100 pixels wide and 1 pixel high spans a square of 100 x 1 pixels.
A similar line would span a square of 100 x 100 pixels if it was turned 45 degrees.