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how does the separation of powers keep
government from becoming too powerful?

So no one central branch has too much power they separate so if one tries to go corrupt the others can stop it

The preamble explains the reasons  why ben franklin and the other founding fathers  of the constitution made our government a republic. by doing this they also replaced the articles of confederation

The president of the united states does not have the authority to “limit” the power of the judicial branch. however, the president can influence the political direction of decisions made by justices on the supreme court through the nomination process when a position becomes available.i would say the best answer is that he can at least interpret the laws