Selling on Teachers Pay Teachers

Dear Rich: I am a teacher who creates many worksheets/handouts and I am looking to sell my work on a site called teachers pay teachers. When it comes to comprehensions or poems, where one creates a worksheet based on a text or written piece, how does it work in terms of copyright? If I find an online article that I want to use for a comprehension to create a worksheet and sell on TpT, and I give recognition to the person who wrote the article, will there be copyright infringement? I've got so many comprehensions that I have created based on articles/texts but not sure how to go about it. Articles published in the U.S. after 1963, are likely protected under copyright and reproducing them is probably an infringement regardless of the attribution. (Articles published from 1923 through 1963 may also be protected.) If you're only reproducing a small portion of the article, it's likely a fair use. If you're not reproducing the article at all -- just providing a worksheet and hyperlink, for example -- there shouldn't be any copyright issues. Keep in mind that as part of your arrangement with Teachers Pay Teachers, you must agree that your "content will not infringe or otherwise violate the copyright, trademark, or other intellectual property rights of any third party."
Who owns your comprehensions? There also may be a secondary issue whether you (or your employer) own the rights to your comprehensions. You probably do ... but the Copyright FAQ for Teachers provided at the Teachers Pay Teachers website explains this and other issues in more detail.