If I can use drawcurve node to draw curve, but what if I want to erase part of curves I drew? I was just reducing strokes, but it wasn't ideal.

Also, after I drew my curve freehand, is there anything I could close the curve?

Many thanks!

Try dropping down an Ends node and choose Close Straight. Depending upon what view angle you drew the original curve in, you may have to chose U, V or both.