“不履行义务”常见的英文表达是 "fail to fulfill/perform one's obligations" 或 "default on one's obligations"。
fail to fulfill/perform one's obligations：强调未能完成或执行应尽的责任。
例句：If a party fails to fulfill its obligations under the contract, it may be liable for breach of contract.（如果一方未能履行合同中的义务，可能要承担违约责任。）
default on one's obligations：更侧重于未能按约定或规定履行义务，常带有一种“违约、失信”的意味。
例句：The borrower defaulted on his loan obligations.（借款人未能履行其贷款义务 。 ）