Hans,
(Assuming most PC based software...)
You can "swipe" a section of any text... that then highlights it. You can press "delete" to remove it.... or right click, and an option box drop down menu appears next to your cursor. Now highlight and click "copy".
That portion of text is now stored in a temporary file.... now go to the area where you wish to insert the text..... right click again and you will have the option of "paste". That now inserts the text copied wherever you have left the flashing cursor. That is how most early word processors worked... and with practice, you can manipulate the posts and your replies quite easily.
Once you copy another lot of text, the memory of the previous text is wiped over.
Keep on doing that to complete the editing as required.
Hope that explains.
Note: - The beauty of the method I show above is ... it automatically puts the quote tags in for you. You merely delete out the other text you do not need.
Perhaps if some are just copying and pasting from the post... that is why the highlighted boxes are not showing to make the "quote" stand out.
Best Regards,
Ian