A reason explains what happened. An excuse asks for permission to let it happen again.

A reason and an excuse can hold exactly the same facts and point in opposite directions. One is offered as description, so it comes attached to what changes next, while the other is offered as a request, aimed at keeping the arrangement exactly as it is. The test is not the content of the sentence but what follows it.

Listen for whether your explanation has a next step attached. If it does not, add one before you say it, even a small one. The difference between the two versions is usually a single clause at the end.