Plain - simple not fancy; not a lot of decorations

    I am going to wear a plain black dress to the party tonight.
    Do you want the plain cake or the cake with the nuts?
    I want just a plain simple phone.
    I do not want an expensive car. I want just a plain car.

Plane - short for airplane

    My sister has a license to fly a plane.
    Did you ever fly in a plane before?
    How big was the plane?
    We are going to New York by plane.

- See more at: http://english-the-easy-way.com/Confusing_English/Plain_Plane.html#sthash.mRd5QyQB.dpuf