What's the different between 不好意思 and 对不起？ Where should I use which?
不好意思 is similar to "excuse me",when you're not really doing something wrong. For example,you can say "不好意思，请问厕所在哪里?"(Excuse me,where is the bathroom?) In this case,you're simply apologize for disturbing others.This phrase is commonly used when talking to unfamiliar people,in order to act polite.
对不起 is like "sorry",when you actually do something wrong. For example,you can say"对不起，我弄坏你的笔了"(Sorry,I broke your pen) In this case,you actually do harm,so you apologize for your mistake. This phrase is mostly used when you want to show others your apology.