What it means
Sorry — for something you actually did. It carries an admission of fault.
When to use it
Don't confuse it with 不好意思 (bù hǎoyìsi), which is a lighter "excuse me" for small things like squeezing past someone.
Examples
duì对bu不qǐ起，wǒ我lái来wǎn晚le了。
Sorry, I'm late.
duì对bu不qǐ起，wǒ我bù不zhī知dào道。
Sorry, I didn't know.
duì对bu不qǐ起，zhè这shì是wǒ我de的cuò错。
Sorry, this is my fault.
The full set of survival phrases for greetings, dining, shopping, and getting around, each with audio.