The word "sumimasen" 済みません has been described as "the most useful word in the Japanese language". It's often described as meaning "excuse me" or "I'm sorry", and it serves quite well to apologize for bumping into someone or to get someone's attention.
But I was puzzled when I bought something at a little shop in a shotengai in the Osaka suburbs and the shopkeeper said "sumimasen". Was he apologizing for ripping me off? I learned later that the word is often used to mean "thank you". We might compare it to "sorry to trouble you".
But it's interesting to look a little deeper. That "-masen" is a negative verb ending, it means something is not done or is not the case. "Nihongo ga wakarimasen" means "I do not understand the Japanese language." (True, and a very useful phrase for travelers!) "Biiru ga arimasen" means "There is no beer." (A very sad sentence, desu ne?)