- I left when he arrived.
Since we don't know how much time passed between the two actions, we could probably think the two action could happened at the similar time. So, we naturally think one action can affect the other, like I left because he arrived. The thing is he arrived first, and after that, I left.
- I left as he arrived.
I think this are the same as #1, but the time gap between the two action is only a very short, compared to #1. And like #1, he arrived first, and after that, I left.
- He arrived and I left.
I think it's like sequencing two different actions briefly so that speaker say something else, which would be actually what the speaker want to say.
- I had left when he arrived.
"Past perfect" here is used to emphasize that one happened, and a while after that, the other happened So, the two action is not connected. We never know if I left because of him, .unlike #1,2 or 3.
- I had left when he had arrived.
By saying two "past perfect", the two actions are not connected. But, I can't think of any situation this is needed.
I think #1,2,3, are interchangeable in almost any case.
I'm sorry that I'm asking to many topics here, but I would only understand by comparing all of these.