One way to remember which word is which is to think of the 'er' in 'paper' as matching the 'er' in 'stationery'. Most simply, stationary is an adjective that means "not moving," and stationery is a noun that means "paper for writing letters."

Stationery refers to commercially manufactured writing materials, including cut paper, envelopes, writing implements, continuous form paper, and other office supplies. Stationery includes materials to be written on by hand (e.g., letter paper) or by equipment such as computer printers.

Stationary is an adjective that describes an object that is not in motion. It can describe a physical object or an unchanging condition. For example, a stationary bike is a bike that does not move, no matter how fast someone pedals on it (at the gym, for example).

noun. sta·​tio·​nery ˈstā-shə-ˌner-ē 1. : materials (such as paper, pens, and ink) for writing or typing.