No matter what you have heard (10%, 300 words, 3 pages, etc etc) there is no set amount of a copyrighted work that can be used before an infringement exists. Each instance is judged on a case-by-case basis.
There are no hard and fast rules on this; the law is intentionally vague. Copying an item directly is infringing on the rightsholder's right to copy, and changing an item is infringing on the rightsholder's right to create derivative works.
Unfortunately there are no hard answers on this in the law; the US Copyright Office explains it this way:
To be copyrightable, a derivative work must differ sufficiently from the original
to be regarded as a new work or must contain a substantial amount of new
material. Making minor changes or additions of little substance to a preexisting
work will not qualify a work as a new version for copyright purposes.
Because preparation of a derivative work is one of the exclusive rights of the copyright holder, you may wish to address the disposition of rights in the resulting work in your licensing agreement.
For more information (if you are in the US) see Circular 14, linked below.
Not really; infringement cases can be brought before any civil court.
Because the vast majority of infringement cases are settled long before they go to court, there is no comprehensive list.
Not necessarily, no, you can watermark a photo that is YOURS, but not one that is already someone else's before, because that would be copyright infringement.
Before an infringement suit may be filed in court, copyright registration is necessary for works of U. S. origin. Bear in mind that a copyright can be filed retroactively, however it will have some effect on the types and amounts of damages awarded.
In the United States copyright law is almost exclusively handled in federal district courts, other than copyright of sound recordings published prior to 1972 and other possible issues that are handled under state laws because they are not pre-empted by the federal laws.
The maximum is 10 years, but most infringement cases are settled long before they reach court, for amounts close to real damages.
Copyright infringement is a violation of federal law, and can carry significant fines. Schools often take it upon themselves to identify, punish, and correct infringing actions before they reach that level. Many schools also address copyright infringement and plagiarism in their honor codes.
The maximum number of songs you can download before being at risk of copyright infringement is 0.
Demo or trial software allows some use before purchase.
Yes, using a newspaper's masthead without permission may constitute copyright infringement, as the masthead is typically considered a creative work that is protected by copyright law. It is important to obtain permission from the newspaper before using their masthead.
Music copyright infringement cases can be brought in civil court, but the vast majority are settled long before they reach that point.
In the US, the copyright holder can sue for up to $30,000 per infringement, or $150,000 if willfull infringement is proven. That being said, most copyright disagreements are settled long before they reach court, for an amount much closer to real damages.