What is the difference between "when" and by the time ?
according to the grammar book.
when = at that time
by the time = one event is completed before another event
But I checked the examples and they seem similar.
Ex: When I arrived, he had already left
By the time he arrived, we had already left.

Could anyone tell me when to use "by the time" ? or those two have exact the same meaning ?

Question

What is the difference between "when" and by the time ?
according to the grammar book.
when = at that time
by the time = one event is completed before another event
But I checked the examples and they seem similar.
Ex: When I arrived, he  had already left
      By the time he arrived, we had already left.

Could anyone tell me when to use "by the time" ? or those two have exact the same meaning ?

Answer

I think for the most part they are pretty similar, but "by the time" usually seems that an action has taken or will take a certain amount of time to be completed. And simultaneously another action will also have been completed or is beginning.

Example
By the time you finish your dinner dessert will be ready.
By the time I arrived at the house, he had already left.

When seems to be more conditional. When is usually followed by a request or some kind of information.

Example
When you finish your dinner, you may have dessert
When I arrived at the house, they greeted me.

But sometimes they are both interchangeable.

Answer

BY THE TIME

There was hardly any guests left by the time we got there. 
我们到那儿时客人差不多都走了。
It was closed by the time we arrived at market. 
等我们到时市场已经关门了。
By the time I got home, she was gone. 
我回到家时，她已经走了。

I hope it helps you!