Why do we replace current sources with breaks and voltage sources with wire? I don't completely understand.

 

This looks like something more appropriate for the homework forum. However, provided you have a grasp of the underlying justification for the use of superposition, you'll be confident that one replaces all sources other than the one currently being considered by sources which are assumed to be effectively inactive or 'dead'.

A 'dead' voltage source is simply one which has zero volts constrained between two points and therefore electrically equivalent to a short circuit between those two points.

A 'dead' current source means there is zero current flowing between two points which is electrically equivalent to an open circuit between those two points.

 

Thank you very much, it is clear now. I thought this forum was for questions about the ebook?