すみません

Using it is easy because it fits almost everywhere. If you bump into someone on the train, say sumimasen. If you want to order a beer at a busy izakaya, shout sumimasen! to the staff. If someone holds the elevator for you, a quick sumimasen works as a polite 'thank you'. It is the ultimate safety net for any social interaction.

Use it when you are out in public. It is perfect for shops, restaurants, and offices. Use it when you are slightly inconveniencing someone. Use it when you are receiving a small favor from a stranger. It is the gold standard for 'neutral' politeness. Even if you forget every other word, this one will save your life.

Do not use it for massive mistakes. If you crash someone's car, sumimasen is too light. You need something heavier like moushiwake gozaimasen. Also, avoid using it with your best friends or siblings. It sounds too distant and stiff. With them, stick to gomen or gomen-ne. Using it with your mom might make her think you are mad at her!

Japanese culture is big on 'meiwaku' or avoiding causing trouble. When you say sumimasen, you are acknowledging that you have entered someone's space or taken their time. It is less about 'guilt' and more about 'social harmony'. It shows you are aware of the people around you. It is the sound of a society trying to stay polite.

You will often hear people say suimasen instead. This is just a slightly lazy, more casual pronunciation. It is very common in daily speech. Men might use sumanai in a more gruff or masculine way. In very formal settings, it transforms into sumimasen deshita to apologize for something that already happened.