In general, "plains" is not capitalized when used in a sentence unless it is part of a proper noun such as "Great Plains" or if it is the first word in a sentence.
The plains in South Texas continue into Mexico.
We took a picture of a Coastal Plains with my friend.
canyon
That's two words, you know.The coastal plain borders the ocean.We studied the coastal plain in school.Our town is on the coastal plain.
The Great Plains of America were once covered by bison.
The rain in Spain stays mainly in the Plains. Last year was the hottest ever recorded.
When you want to bump your sentence up a few adverb notches. The wolf warily patrolled the plains, using its cunning to locate and brutally vaporize intruding ostriches.
"The wind blew endlessly, making her detest the barren plains she had agreed to move to."
The sentence segment "Standing 850 feet above the plains with a circumference of five miles" contains a punctuation error. To correct it, a comma should be placed after "plains" since it is the end of a descriptive phrase that needs to be separated from the main clause.
This is not an English word, but it means the South American treeless plains. We traveled to the llanos last summer.
Pampas is a plural proper noun referring to the region of grassy plains in northern Argentina; use it the same way you would use "Great Plains." In a sentence, one might say:"The Pampas include thousands of square miles of land in Argentina."or"Gee, that was a fun vacation we took to the Pampas."