What’s the difference between these sentences?

[blissful]

Hi

What’s the difference between these sentences?

1)Simon will leave when Alan has arrived.

2)Simon will leave when Alan arrived.

[5jj]

The first is correct; the second isn't.

[blissful]

Hi

How about changing to “Simon will leave when Alan arrives”

Is this sentence correct?

[5jj]

How about changing to “Simon will leave when Alan arrives”

Is this sentence correct?

Yes.

So are, "Simon left when Alan had arrived" and "Simon left when Alan arrived", though these two refer to past events.

[blissful]

Hi

Thank you for your reply.

What’s the difference between 1 and 2 and 3 and 4?
Any change in meanings?

1)Simon will leave when Alan has arrived.
2)Simon will leave when Alan arrives.

3)Simon left when Alan had arrived.
4)Simon left when Alan arrived.

[5jj]

In #1 and #3, Simon's departure may happen after Alan's arrival. #2 and #4 suggest that the two events happen at the same time.