Situations in which to use ni and nin?

您 is the respectful form of 你。You may be expected to use 您 when you are addressing people listed below:

• Old people or elders whether you are acquaint with or not. E.g. your father or an elder stranger whom you want ask the way in the street.
• Teachers (if you are a student); High-ranking officers (if you are an employee).
• Anyone you want to show your politeness especially in the first meeting.

P.S.It's quite common for people to use 您 to show their politeness in northern China. On the contrary, people in the south of China prefer to use 你 most of the time. Because the character 您 , which develops from the north dialect, never exists in most of southern dialects in China.