The pen tool is evil!

Not really, it just takes a bit of getting used to.

If you just click the pen tool a few times on the artboard, you create an number of anchors (dots) and each time you click a new anchor – it’s like you’re drawing connect-the-dot style.

If you click back on the original anchor, you’ll create a full ‘shape’.

If you don’t want to complete the shape, just press command as you click to create your last anchor point or press enter after creating your last anchor point.

If you click, hold and drag using the pen tool, you will create handles.
Handles allow you to create curves between the 2 dots/anchors.
You can move the handles up, down, left and right.

If you hold down the ‘alt’ key and click on an anchor point – you can go back through and change the curves you made

When creating an arc or Arch Using the Pen Tool

– Use 3 anchor points
Bottom right, the top and bottom left of the arc.
– hold down shift as you’re making the second anchor point at the top of the arc:

 

Holding shift when adjusting the handles is also really a good way of going back and adjusting the handles to fit on a curved line: