You can transition from sketching a line to sketching a tangent arc,
and vice versa, without selecting the arc tool.
To autotransition between line and arc:
(Sketch toolbar) or click Tools,
Sketch Entities, Line
and sketch a line.
Click the endpoint of the line, then move the pointer away.
The preview shows another line.
Move the pointer back to the endpoint, then away again.
The preview shows a tangent arc.
Click to place the arc.
Move the pointer away from the arc endpoint.
The preview shows a line. You can draw a line or change to an arc
as in Step 3.
To toggle between line and arc without returning to the endpoint,