Let’s say I am missing my novel and I think that my friend might have taken it after I left the place.
1: By the time I left my apartment, my friend was still there, so he might have taken it.
2:When I left the apartment, my friend was still there, so he might have taken it.
I know what 2nd expression means, but not so sure about the first expression. Does it convey the same meaning as the 2nd expression?
In this context, the term when points out a specific point in time (when you left the apparment), while the phrase by the time… suggests a passage of time that took too long.
So your 2nd sentence (when I left…) better conveys what you are trying to say. The first sentence (by the time I left…) doesn’t really convey the correct circumstances of your assertion.
The injuries didn’t look too serious, but by the time the ambulance arrived, the victim was dead.
(Specifies that the delay was at issue)
The injuries didn’t look too serious, but when the ambulance arrived, the victim was dead.
(Still technically correct, but missing the passage of time as an issue)
Sometimes that context is important, so the two phrases are not always interchangeable.
When you turn on the switch, the light goes on. — correct
By the time you turn on the switch, the light goes on. — doesn’t make sense
Source : Link , Question Author : Thor , Answer Author : Robert Cartaino