1.It doesn't matter常用于别人做了对你或多或少有一些不利或者影响到了你,向你道歉时,可以这样回应对方的道歉.
2.回应对方的谢意的时候,说“没关系”常用 Don't mention it.you're welcome.
3.I can not find key to the office.回应这句一般不用It doesn't matter. 用Don't worry.之类的比较好.还有这句的key前是不是少打了个定冠词the.