This reply was UNHELPFUL because it is so unclear as the wording is inaccurate; therefore hard to follow.
What it should have said was for copy/paste right click for plain text:
Inbox> (open any email you want to reply to) >(...) (at extreme right of the reply arrow)(more actions) View>Open in new window
After hours of reading people's comments/ suggestions and inaccurate advice I found a way for plain text:
1st: to be able to simply copy/paste plain text in an email using right click:
(next to 'Home' on Task bar at top left) View> View settings>Email (below Layout)>Compose and reply>Pop out settings> pop out to a new window
I've noticed that if I open a new email from Home it will pop out, then just add To (recipient), the draft will save & when I open it again it will open fully, ie not in a pop out (for those who hate pop outs!) It will still copy and paste with right click.
2nd: to enable spell check to work again using two languages at the same time ie in the same email:
(next to 'Home' on Task bar at top left) View> View settings>General (below language & time) >Privacy and data>Privacy settings>Turn on experiences that analyse your content (toggle ON) Close
I read someone saying they did not want MS to be doing this and an IT expert (I'm afraid I can't remember who) replied that they were surprised that this was not ON anyway, as for things such as spell check & predictive text, in other apps it is automatically already set as ON.
I hope this helps one person at least :)