Well, there is the 'Quote' button on the desktop site. That is how you can do so in the thread itself.
On the Forums app, either AC or Tapatalk, you can click the post to highlight and there is a button labeled Quote on the AC forums app, or a couple of comic book balloons on Tapatalk.
Desktop you can highlight what you need to copy and right click. On mobile, I think you need to start quoting in order copy, which is a long press of a word, and you can highlight what you need, and there should be a copy button toward the top.
However, I do agree with Javier P. that you should attribute proper credit when you copy/paste.