"Here is a simple rule for being happy:"

Subject: A simple rule
Verb: is

Right , this sentence has an inverted structure
The verb is 'IS'. 'HERE' is an adverb : it tells us WHERE the simple rule is.

Suppose the sentence had a phrase such as 'IN THIS BOOK' instead of 'HERE'. The sentence would then be "In this book is a simple rule for being happy".

To find the subject, remember that the subject must agree with the verb. If you make the subject plural, the verb must become plural too (and vice versa).

See these for example:
"Here ARE some simple RULES for being happy"
"In this book ARE some simple RULES for being happy"
See this too:
"In these BOOKS IS a simple RULE for being happy"

Here are some sentences without the inverted structure:
"This book contains a simple rule for being happy" (Subject is 'THIS BOOK')
"A simple rule for being happy can be found here" (Subject is 'A SIMPLE RULE...')


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