Although these two words sound alike, they have very different meanings.
Stationary means "staying in one place". It indicates that the subject is not moving or some measure/quantity is not changing.
From this distance, the satellite appears to be stationary.
Stationery means "writing material".It includes anything related to writing. The wedding cards,greeting cards, writing implements,pencilcase etc.; all form part of stationery.
A hotel often provides stationery with its name pre-printed.
Straight is most often used as an adjective meaning "unbending".
The path cut straight through the woods.
Strait is a noun which means "a narrow passage of water connecting two large bodies of water" or "distress, dilemma".
He was in dire financial straits.
The cars remained stationary for almost an hour.
They can't keep the dogs stationary for long.
The shop near the school provides all the stationery items.
The teacher asked the students to bring the stationery required for the drawing class.
Her house is just at the end of this straight road.
He never talks straight.
This strait will lead you to the other side of the garden.
They have entered the straits of financial crisis.