I am so sorry - You use this for serious mistakes or when something serious happened to someone. For example, say a person's dog passed away, you can say "I am so sorry for what happened to your dog." Or if you make a huge accounting error on someone's bank account, "I am so sorry I accidentally withdrew a million dollars from your bank account!" Sorry - You can use sorry for almost anything. For example, if you bump into someone or if you accidentally spilled milk on someone. Excuse me - You can use "excuse me" almost interchangeably with sorry. However, there are some specific instances that would make more sense to use excuse me. For example, say you are in a crowded place like a movie theater or there is a person blocking your path in a tight hallway, you can say "excuse me" for them to move out of the way. Can I enter/ May I enter: You can use this whenever you are going in someone's private room, house, or building. Message me if you need me to elaborate more