The correct phrase is "On the same plane". A plane in this case being a level of development, existence, or achievement.
yes
Yes, they are the same thing
plane, plain, plain
The North China Plain is also known as the Great Plain of China. Great China Plain is not a name that is commonly associated with the place.
No
No
The correct usage would be "on the same plane". Plane as in "plate", "table" or "page". These are nouns. "Plain" is an adjective.
Unattractive, knit, prairie, unadorned are synonyms for the various meanings of "plain".
yes. They are the same thing. Plain flour is an Australian term where all-purpose is the American.
Plain yogurt and taditional yogurt are mostly the same. Traditional may have a slightly sweet or vanilla flavor or they might be exactly the same depending on the brand.
Its the same thing as a plain pizza.
No! While some deserts do have vast plains, a plain can also be a savanna or grassland.