I have a sentence with the bit "a now forgotten era".
Would it be written correctly this way or should it be hyphenated as "a now-forgotten era"?
What about "I dedicate this board to Atlantis, a once powerful civilization"? Should it be "a once-powerful civilization"?
Ah, hyphens. I used to read a lot into these topics too, so I'll tell you what I've learned through other people and my own experience.
When an adjective phrase is formed using an adverb and an adjective there is no need for a hyphen. In your sentences, "now" and "once" are adverbs, so there is no need to hyphenate: A now forgotten era, a once powerful civilization.
If you're not one to consistently follow strict rules about grammar, think of hyphens as an aid for clarity. Only use them when it makes the sentence less confusing. This is mostly when adjective phrases use two nouns or adjectives, for example: "A chocolate-filled pastry". Even though people will understand "A chocolate filled pastry", some odd minds will read that the chocolate filled the pastry.
To sum it up, no need to hyphenate with adverb-> adjective. Your sentences are fine without hyphens.
As has been the case with commas, 21st century usage generally shows a preference for reduction of nonessential punctuation...hyphens included. This isn't a pat grammatical rule. Most hyphenated modifiers are, in fact, adverb-adjective pairs: an ever-present danger, an oft-quoted passage, etc. First, let's look at where you definitely would NOT use a hyphen:
This civilization, while once powerful, is now forgotten.
A hyphenated modifier goes BEFORE a noun. The implication is that the two words work so closely together that they create a unitized meaning as a single modifier. Even so, I would not use hyphens where the meaning is clear without them. Their most justifiable use is in resolving ambiguity.
