The Fair Use Network details the legal criteria in determining fair use, for example, a blogger posting content from a newspaper article.
1. The purpose and character of the use â┚¬â€ whether the new use is commercial, nonprofit, or educational, among other relevant characteristics; and whether it is “transformative” or merely substitutes for the original.
Adding your thoughts and comments to the copied material can make it transformative vs. a straight copy with no comments, which is not. Short quotes are always better.
2. The nature of the original work â┚¬â€ whether the original work is factual or creative; published or unpublished; in or out of print.
Factual and/or published generally tilts towards fair use, creative and/or unpublished doesn’t.
3. How much, and how substantial a part, of the original was taken â┚¬â€ whether more was taken than needed, or whether the “heart of the work” was taken.
The “heart” of the matter can be quite small and still be considered NOT to be fair use.
4. The effect of the new use on the market for or value of the original work â┚¬â€ whether the new use substitutes for the original work in the commercial marketplace, or reduces its value by supplanting demand for the original.
This is the big factor. Does your copying mean the original author could lose income or could there be less demand for it? If so, it’s probably not fair use.
EFF also has a useful explanation of the the 4 criteria. They note that noncommercial use is more likely to be fair and linking back to the original helps reduce the substitution effect. Linking back, in my view, should always be done, both for ethical and legal reasons. EFF also explains that simply because a site says, for instance, you can only copy a few lines, that would be their guideline, and is not the law.