I couldn't get to school on time和I didn't get to school on time有什么区别?

I couldn't get to school on time和I didn't get to school on time有什么区别?
couldn't 和 didn‘t 不能互用.
I couldn't get to school on time.
句意是：“我不能按时到校”.(couldn't do sth没能力做某事).
I didn't get to school on time.
句意是：“我没有按时到校”.(didn't do sth过去没有做某事)