Legal analysis: Buying and selling does not break the lease, and mortgage does not break the lease. The rental contract is not affected by changes in the property rights of the house. The house mortgage does not require the tenant's signature, and it has no legal effect even if the tenant signs it.

Legal basis: "The People's Republic of China and the Civil Code"

Article 490: When the contract is established, if the parties enter into the contract in the form of a contract, it shall be signed by both parties. The contract is established when the signature is stamped or fingerprinted. Before signing, sealing or fingerprinting, one party has performed its main obligations and the contract is established when the other party accepts it. Laws and administrative regulations stipulate or the parties agree that a contract should be concluded in written form. If the parties do not use written form but one party has performed its main obligations and the other party accepts it, the contract is established.

Article 502: A contract established in accordance with the law shall take effect from the time of its establishment, unless otherwise provided by law or otherwise agreed upon by the parties. In accordance with the provisions of laws and administrative regulations, if the contract must undergo approval and other procedures, the provisions shall be followed. If the failure to go through approval and other procedures affects the effectiveness of the contract, it will not affect the effectiveness of the contract's obligation clauses such as submission for approval and related clauses. If the party that should go through the application approval and other procedures fails to perform its obligations, the other party may request that it bear liability for violating such obligations. In accordance with the provisions of laws and administrative regulations, if the modification, transfer, termination, etc. of a contract require approval and other procedures, the provisions of the preceding paragraph shall apply.