duì lǎo shī, zhǎng bèi, kè hù hé chuāng kǒu gōng zuò rén yuán, yòng nín tōng cháng bǐ jiào ān quán

对老师、长辈、客户和窗口工作人员，用“您”通常比较安全。

For teachers, elders, customers, and counter staff, using 'nin' is usually safer.