Stationary with an A means immobile or unmoving. The strong anchor kept the ship stationary despite the rough seas. George remained stationary so as not to attract the attention of the angry bees.
Stationery with an E means writing paper or office supplies. I received a beautiful stationery set for my birthday. The stationery cupboard at the office was running low on supplies today.
I have an unusual way of keeping these words separate, but it seems to work: the letter A has two legs and is stationary; the letter e has a rounded bottom and is not.