Trim off part of shape?

I know this is easy, but...

How can I trim off part of a shape? For example, I have a circle and want to trim off the top 1/4 of it. When I try to do this, I end up getting a line that closes the circle back off. What am I missing here?

Draw a "subtract" rectangular shape that overlaps and then, optionally, choose Combine to make the results permanent.