Sumimasen

すみません

Sumimasen is one of those convenient expressions with a lot of different meanings depending on how you use it. It can mean, "I'm sorry," "thank you," or "excuse me." I don't know how accurate it is, but I usually think of all of these as different ways to say, "I'm sorry." You can use it directly to mean, "I'm sorry," when you are apologizing for something. Sumimasen is more formal than "gomen nasai," which also means, "I'm sorry."  In this case, you might say it in a very heartfelt way. If someone does something for you, you can say it to mean, "thank you." In this case, as well, you could think of it as meaning, "I'm sorry you had to go through the trouble." In this case, you would say it in a grateful way. You can also use it to call someone or get someone's attention. In this case, you can think of it as meaning, "Sorry to bother you." You can say it politely or yell it out, depending on the circumstances.

Shuzo tells me that this expression comes from the word, "sumu," which means to be clear. Sumimasen would be the negative form of that. You can think of it as meaning, "My conscience is not clear because I have troubled you." An alternative explanation is that it comes from "sumu," meaning to end. The negative of this would be that, "it's not over between us." This comes from the Japanese idea that, if someone does something for you, you are obligated to do something in return. In other words, you can think of it as, "I owe you one."