The words plane and plain sound similar when spoken, but are different words.
The word plane can be used both as a noun and as an adjective.
As a noun plane means: airplane; aircraft; jet; seaplane.
As an adjective plane means: flat; even; smooth; level; horizontal.
The word plain can be used both as a noun and as an adjective.
As a noun the word plain generally refers to an area of land that is flat and has scanty or no vegetation.
The word plain as an adjective has multiple meanings:
(a) When used with reference to the appearance of somebody or something plain means:
simple; basic; natural; not beautiful; homely; ordinary. Antonym: striking; elaborate.
(b) It can also be used to mean: Clear; evident; obvious; readily understood; apparent. Antonym: ambiguous.
Examples of plane as a noun:
1. The company’s President used his own private plane whenever he wanted to travel for business purposes.
2. The brothers decided to start a small charter business ferrying passengers in their two-seater planes.
3. The coastal authorities purchased two new seaplanes to increase security after repeated pirate attacks against local fishermen.
Examples of plane as an adjective:
1. At the end of the third round, the chess players were both on an even plane.
2. The architect explained the advantages of setting up the small structure on a plane rather than in the original place.
3. During the air show all the jets came on an even plane to make a perfect formation.
Examples of plain as an adjective:
(a) With reference to appearance/looks:
1. Although Shamita was plain looking, she surprised everybody by winning the college beauty competition.
2. The costume was plain but suited the actor perfectly for the role.
(b) When used otherwise:
1. It was plain that Meenakshi was not really as knowledgeable as she claimed to be.
2. The lawyer presented good proofs that made it plain that his client was innocent.
3. It is a plain fact that honesty and integrity will get you ahead in life no matter what others say.
Examples of plain as a noun:
1. The hotel owners decided to set up a small ring for horse riding for kids on the plain near the premises.
2. The plain was an ideal place to fly kites as there were no electric wires overhead.