Can the pen draw in a straight line.?

If you mean the Pen Tool, do this:

-Set your brush to the desired thickness

-Drag a guide to where you want your line to be

-Place your points along it to the desired length (or height if it's vertical)

-With the Pen Tool selected, right-click, and from the dropdown menu choose Stroke Path.

Do you mean the brush?  If yes, then try holding down Shift.  In the full version shift forces the brush to move perpendicular to the screen, or at 45°.  Touching down with the brush, and them holding down shift while touching down at a different location will force a straight line between the two locations.