DESCRIBE THIS GRAMMAR : Rarely do networks show nature documentaries on prime-time television.?

Question

what happens to the subject and verb? when is do rquired?

Answer 1

When you begin a sentence with the word rarely, you need do after it, and then the subject (networks) and verb (show).

You could rewrite it, in a different order, that doesn't require do:
Networks rarely show nature documentaries on prime-time television. (In fact, this is how it is normally said. Your example is a bit more "literary")

The same goes for "Never" and "Hardly ever":
Never do I get the chance to watch TV.
Hardly ever do I see my friend Michael.

Answer 2

It is, very obviously, a question containing two examples of adjectival nouns. In "nature documentaries" the noun "nature" is used as an adjective to describe the type of documentary. In "prime-time television" the composite noun "prime time" is used to describe television. The word "prime" can be either an adjective or a noun in English. Here is a link showing the various possible meanings. http://www.answers.com/topic/prime.... Here the meaning is "optimal" -- the time when television is viewed by the largest number of viewers.