First: INAL (I’m not a laywer.)
I have, however, read and closely followed copyright for a decade or more.
Here are some quick, general points:
“Prior to what year are works in the public domain?”
1923. (It’s 2008 now, and with extensions, copyright attaches for 95 years.) In general, for works created on or after January 1, 1978, the term of copyright is the life of the author plus seventy years after the author’s death. (See page 4 in the circular listed below.)
“Only published works can be copyrighted.”
Wrong: Copyright attaches the instant you have created the item. The item must be in tangible form (ie, ideas cannot be copyrighted.) That is so fundamental that it’s the subject of the very first paragraph of the very first circular from the copyright office.*
“So why register a copyright and pay the fee?”
Because without that, you will not succeed in getting damages for infringement in court. (You can sign up for electronic submission and registration, and submit images in bulk for one fee. Doesn’t make it easy, but certainly makes it easier than it was.) Without registration, your remedy is to stand on the court steps and stamp your feet.
“Can I just mail myself a envelope and use the post office time on the cancelled stamp?”
Sure… but you can also stand on those same court steps, and whistle “Dixie” and get the same legal effect: none.
“The first four bars of music; the first 2 sentences; the (other magic pixie dust) are can be used without infringing.”
See “whistling Dixie” above. (That’s nonsense.)
“What about fair use?”
Whole can-o-worms. The basic answer is: Ask the copyright holder. To cover fair use would take more than a few dozen paragraphs. In a nutshell, it’s not cut-and-dry and is settled in court, on a case-by-case basis (and case law is all over the place on it.) Some bits of it are reasonably clear (such as using copyrighted images in an actual educational institution [but if you print them in a PDF which can be taken off-campus, the ice just got a lot thinner]). You may quote or reproduce for legitimate criticism. (But you can see how vague words like ‘legitimate’ can lead to sticky situations. Like I said: can-o-worms.)
Basically, if you’re claiming fair use (like Richard Price or Jeff Koons) instead of getting permission from the artist first, be prepared to defend yourself in court. Don’t say I didn’t warn you!
More info: https://www.copyright.gov/fair-use/
“What is proper copyright labeling?”
© year name (or Copyright[or Copr.] year name)
*But please, don’t take my word for all this. Instead, start here:
That’s the basic, fundamental US copyright circular, and it is the entrance portal to the rabbit-hole that is copyright law.