It depends on the context, you could have either... I reached an understanding with Tommy not to fight any more. Tommy and I reached an understanding to stop fighting.
The study of physics includes the study of how objects move and react to each other and the study of heat and the properties of light. All of these are important in different forms of science.
The understanding of the cell is the key to comprehending biology because we are made of these cells, and biology (having the word "bio") means learning about our bodies and how amazingly they work.
Wow - good question - basically it's because of the concept of power in this context. It doesn't mean electrical power or energy - it refers to the power to govern. This kind of question is really...