对不起。

Use it when you are late for a coffee date. Use it when you accidentally step on someone's foot. You can place it at the start of a sentence. For example, 对不起，我忘了 means 'Sorry, I forgot.' It is a complete thought on its own too. Just say it with a small nod. It shows you take responsibility for the hiccup.

You should use this in most everyday social mishaps. Use it at a restaurant if you spill water. Use it in a meeting if you interrupt someone. It works perfectly when texting a friend you've kept waiting. It is the 'safety' phrase for any social blunder. If you feel a tiny bit of guilt, this is your go-to.

Don't use it to mean 'Excuse me, may I pass?' For that, use 请让一下. Don't use it to ask for a favor or get attention. That is usually 不好意思. If you are at a funeral, 对不起 might sound like you caused the death! Use words of sympathy instead. Avoid it for very minor things like a tiny sneeze.

Chinese culture values 'face' and social harmony. Saying 对不起 restores that balance. Historically, it implies you are too ashamed to look them in the eye. In modern times, it is less dramatic but still very polite. It is a bridge that mends small cracks in relationships. It shows you are a person of good character and 'ren' (humanity).

If you want to sound extra sorry, add 非常 (fēicháng) in front. That makes it 'Very sorry.' If you are talking to a boss, use 真对不起. For tiny mistakes with friends, many people swap this for 不好意思. It's like the difference between 'I'm sorry' and 'My apologies.' Both work, but one feels a bit more official.