Administrator

The steps to trim a circle by a line should be:
1. activate the Trim tool
2. click on the line, which then changes color to a dark olivegreen
3. click on the circle, near the intersection with the line, on the side which should be kept
4. possibly you have to click once again on the circle, near the other circle/line intersection
The trim tool consider a couple of rules when trimming circles. Thus it can happen, that you have the half circle after the first click on the circle and sometime you need a second click on the opposite side. This depends on the angle of the line, if you want to keep the upper/lower or right/left part of the circle and near to which intersection point you click first.
When I say near to intersection means, that the tool calculates the distance from the click point to both intersection points and then chooses the intersection which is nearer to the click point. When you click the circles circumference somewhere in the middle between the line intersection, you possibly can't see which intersection is selected by the tool.
For reference you can do this:
 select the circle before trimming and you see 4 blue squares on the circumference, at 0°, 90°, 180° and 270°.
 after step 3., if you still have the full circle, press ESC to cancel the trim command, then select the circle again. Now you see only one square on the circumference, at the line intersection point.
This is because the tool transforms the circle into an arc of 360°, circles and arcs are different entities for a CAD system. Press CTRL + Z to get the circle again.
My observation is, that you have to consider the direction of the circle, which is counterclockwise in CAD.
When you first click near the intersection which will be the starting point of the resulting arc, in sense of CCW direction, then you have to click a second time near the end point. This may be helpful to trim a cake slice out of a circle.
But when you first click near the intersection which will be the endpoint of the resulting arc, in sense of CCW direction, then the circle is cut at both intersections.
Another option is, if this is too confusing, to use the divide tool from modify command.
 Activate the divide tool
 select the circle to divide it
 with intersection snap on, click on the first intersection point
 select the circle again
 with intersection snap on, click on the second intersection point
 now you have two arcs and you can delete one of them
