鍵をかけておけば、どろぼうに入られません。Why not かければ?

[GenjiMain]

In the sentence 鍵をかけておけば、どろぼうに入られません, my guess is that the part in bold is combining かける + おく. If so, can someone explain the logic of this? Why is the phrase not 鍵をかければ、どろぼうに入られません?

Thanks.

 

[WaterFox]

鍵をかけておけば、
When you locked the door in advance,
When you keep the door (being the "state" of ) "locked",
The expression is talking about the door being locked, the state, the situation.

鍵をかければ、どろぼうは入れなくなる。
When you lock the door,
The expression is talking about the "action" of locking the door.
It refers to the "change" from "unlocked" to "locked."