How does the legislature branch control the powers of the president?

The President exists to execute the laws that the legislative branch passes. If the legislative branch chooses not to pass a law that would give the President a power over something he wants, then he does not have that power. If the President does something that is not expresssly forbidden by existing law, Congress can pass a law that does expressly forbid him/her from doing it. If the President wanted a particular type of law passed, like a tax cut, Congress could refuse to do it because only Congress can authorize a tax cut. Also, Congress is the branch that appropriates money to be used by the President to do things. This is called the "power of the purse". If Congress does not give the President the money to do something, then he cannot do it. For example, if the President wanted a one million soldier army at all times but Congress felt that 500,000 soldiers is enough, Congress can control the President's wishes by appropriating only enough money for 500,000 soldiers. The executive branch has the power to do things, but the legislative branch has the power to limit the things he can do.