There is a fine line between infringement and not. It gets blurrier by fair use, fair dealing and other exceptions.
For example, me quoting David's answer verbatim, in part or in total, would be per se copyright infringement. If I use that to criticize one point of the answer and I take no more than necessary, my infringement is fair use - and not to be treated as an infringement.
And then there is academic quotation style, bypassing the need for actual citation or copying: David says in his otherwise excellent answer, that fair use isn't ever an infringement on the copyright. However, fair use is used as an affirmative defense most often - and some few times, something that was thought to be fair use isn't, and thus it is still an infringement. For example, Good Mythical Morning took a commercial photo of a prison and discussed the depicted prison, but never the photo. That is not criticism of the photo and not transformative to the photo. As photo licenses for the photo are available they infringed on the market, the work was a creative photo, so it was copyright infringement. That example case by the way was ended of an error of the lawyer - not an error of judgment.