Sketch fix element on background elements

[google_msn]

Hello, 

I wanted to ask how/if you can fix elements on elements in the background, when they are not real elements, for example in this picture the circle that is not really a circle, but it would still have a mid-point that I can't access.

Or the line: Why can't I bind the line to the outer body line?

Do I have to make other sketches to achieve that?

I'm a beginner in CAD, so please don't blame me for asking simple questions... 

[davebYYPCU]

You can only snap to current sketch items.

older sketches - you need to Project > Project or Intersect, to bring items into current sketch.

Might help....

[google_msn]

Thank you very much that helped indeed.

[jiang_peng]

Please try turning this option on if you want to snap to the background elements automatically: