Excuse me or sorry? A question of before and after

'Excuse me' is a way of verbalising a tap on someone's shoulder.

We use the phrase ‘excuse me' when we want to warn someone of something. Getting off the bus, we say ‘excuse me' before we push the passenger who is blocking the way. If that person fails to get out of the way and we do push, we say ‘I am sorry'.

So, ‘excuse me' is something we say before and ‘I am sorry' after the harm is done.

If you haven't paid attention to what was said, or if you couldn't hear it, you can say ‘Sorry?' with your voice going up. This means, ‘Oops, I didn't catch what you said, could you repeat it, please'.

If in the same situation you say ‘Excuse me' (with some intensity and perhaps a stern look), it means ‘How dare you say that'.

'I apologise' is a more formal way of saying ‘I'm sorry'. That would sound a bit funny on the bus, but perfectly natural at a formal meeting: ‘I apologise for being late'.