plain, plane

Plain or plain means not fancy; unadorned, or without anything added--
He only likes plain hamburgers.
The plain truth is that it looks awful.
The Amish are also called the plain people.
She always considered herself a plain woman, but he thought she was pretty.
Plain also means a flat field--unobstructed, unadorned by trees, "plain"--
"..above the fruited plain."
The great plains of the west grow much of our wheat.
"The rain in Spain stays mainly in the plain."
Plain is used to mean obvious or clear--
It became plain that the plan would not work.
It is plain to me that we are not getting a raise after all..

Plane or plane ( rhymes with plain ) usually means an airplane--
Are you taking the train or a plane?
"Trains, and boats and planes, can mean a trip to Paris or Rome."
Plane also means a woodworking tool, one that shaves off slivers of wood--
He fixed the door with a plane.
To plane, a verb ( action word ), means to use the woodworking too--
He planed the top of the door, but it still won't shut.
Plane may also mean a level, as in a "moral plane" or an "astral plane"--
Some amateur philosophers believe that they inhabit a different astral plane.
Plane is used as a math term, meaning all of the points on one flat level or "plane"--
The only poor grade he got was in plane geometry.

Now that you know that, you can say--
"From the plane, we could see the plain below, like a patchwork quilt."
"Standing on the plain, we could see the plane flying overhead."
"It was plain to see that she was afraid to get on the plane."

Note--"planed" is the past tense ( yesterday, or some time ago ) of the verb ( action word ) "plane"--
He planed the door so that it wouldn't stick.
"Planet" means Earth, or another celestial body, out in space or orbiting the sun--
NASA will soon have video from another planet to show us.