How do you explain the difference in meaning between a) By the time I arrived, Jim had already left.

b) Jim left before I arrived.

Please email me....
Hello, Jason—and welcome to English Forums.

There is no difference in meaning. A and B are simply two different structures for saying the same thing.

As Mister Micawber explaied, both of the sentences have identical menaing.

When you say the events in order they happened(chronological order), you can simply use past simple.

But when there are two events and you want to talk about the second one before the first one, you have to use past perfect for the first action to clearify that it has happened prior to the other event.

I hope you don't fine my explanation confusing.