English has some apologetic terms(表示歉意的用语). It seems that they mean the same thing. They almost do. However, what's the difference between "Excuse me", "I'm sorry", "Pardon" and "What"?
Pardon
Americans don't really use" pardon". If you do, they will probably joke that you're trying to sound pretentious(炫耀的) or joke that you're British.
I'm sorry
We often use "I'm sorry" for past events. That's to say, it is generally used to apologize for something you have already done. For example, if you step on someone's feet on a crowded bus, you would say "I'm sorry" or "Sorry".
Excuse me
This is generally a term to ask permission(允许) for something. For example, you're walking through a crowded space and someone is in your way. You would say" Excuse me' to ask him to move out of your way.
What?
Americans usually use it to ask someone to repeat himself. If you want to be polite you could say, "I'm sorry, what was that? "or" Excuse me, what did you say?" or any other such thing. Most Americans will just say "What?" though.
In our daily life, we can also use these terms by mixing them up. You could, for example, ask someone to move by saying "Sorry, would you mind moving?" It's an extra-polite way to make your request.