The door is locked. How to open it from the outside?

The door is locked from the outside-usually divided into two situations: one is locked from the inside, and the outside can be opened with a key; The other is that after the lock is locked inside, it can't be opened with a key outside, that is, there is only a small lock inside, and there is no keyhole. Methods: 1. If the door can be used inside and outside, it can be opened. 2. If the door is double-secured or there is only a small lock inside the door, and there is no keyhole, once it is locked, it cannot be opened outside.