すみません as thank-you

When someone holds a door for you, picks up something you dropped or moves so you can get past, you can say すみません instead of ありがとうございます. You thank and apologise in the same word: they went to trouble for you, and that is what you are putting into words.

ありがとうございます is still right when you have received something — a present, help you asked for, an answer to a question. すみません fits when the other person did something unasked and you feel you caused a little bother. Both work; the difference is where you put the weight.

You will also hear すみません when someone wants attention — in a restaurant, in a shop — and when someone bumps into you. One word, three uses: excuse me, sorry, thank you.