Final answer:
The president can veto a bill that Congress passes if it is strongly disagreed upon, which sends it back to Congress with a veto message. An override of a presidential veto requires a two-thirds majority in both the House and the Senate. The veto threat can also compel Congress to compromise on the legislation.
Step-by-step explanation:
If the president strongly disagrees with a law that Congress has just passed, the most definitive action the president can take is to veto the law. This means the president rejects the bill by sending it back to Congress with a veto message outlining objections. Congress can attempt to override the presidential veto, but this requires a two-thirds majority vote in both the House and the Senate. The threat of a veto often encourages Congress to make compromises to ensure the bill is more acceptable to the president.
The president may also use a 'pocket veto' if Congress adjourns within ten days of sending the bill. Moreover, while the president does not have the power to propose amendments before a law takes effect or take legal action against a law, he or she can use signing statements to express opinions on the constitutionality of a bill's provisions.