This is not such a big mistake to make, but it is still something that is quite easy to mix-up!
Stationary means standing still or not moving
I thought the train was moving, but when I got closer I realized that it was stationary.
Stationery means writing paper, especially the kind with matching envelopes. It is also used to talk about writing and other office materials in general.
I just got new stationery with my name on it! I can’t wait to send letters to all my friends!
I need to find a good stationery store near my house. It takes to long to travel into the city to get my office supplies.
So, keep this in mind, especially when writing :).