excuse me

Both are used to admit guilt or intrusion. Excuse me, but may I pass through this line? Excuse me, but I didn't understand the question. I'm sorry that I stepped on your foot; please excuse me.
I'm sorry is also often used to express regret, but not guilt. I'm sorry to hear about your father's death. I'm sorry he feels that way. I'm sorry they won't be able to come to th

PhilipThis is not an exhaustive explanation, but I think it's a good start.

Yes, very much. Thank you,Philip.

Please help me with this.

PhilipExcuse me, but may...

Is but a part of a standard expression? Please tell me why it is used here.

vsureshIs but a part of a standard expression? Please tell me why it is used here.

It's a conjunction to add a contrasting point of view. I don't like movies, but I love live theatre. I don't have two brothers, but three.

Thank you, Philip.

PhilipI don't like movies, but I love live theatre.

Yes, here it is clear: But is used to denote the contrast between two and three.
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Please tell me if I have understood this.

"Excuse me,but I am unable to understand your question."

vsureshIs but a part of a standard expression?

Right. It's a common turn of phrase. Excuse me, but ... I'm sorry, but ...

Excuse me (for interrupting), but (I need to interrupt because) ... can you tell me how to get to the train station?

CJ

Thank you very much, CJ.