如果一方不履行合同 generally speaking a contract cannot be changed after it has been signed by both parties; one party is entitled to cancel the contract if the other side cannot execute it
如果一方不执行合同 this contract will come into force as soon as it is signed by two parties
不履行合同 back out of contract; back out the contract; breach of contract; non-execution; non-performance of the contract; nonperformance of contract
履行合同 comply with an agreement; fulfil a contract; performance of a contract [obligation]; carry a contract; discharge a contract◇履行合同保证人 performance guarantee; 履行合同风险 performance risk; 履行合同义务 obligation of contract; 履行合同责任 burden of contract