You'll often see a message where someone has quoted the previous poster, then interspersed their comments among the quoted material.
How do you do that here? Easy.
Second, Hit the 'Quote' button. That brings in all the text.
Third, select all and remove formatting - that's the Aa button beside 'Arial' and 'Size'.
Now it's plain old text.
Edit out the cruft you don't want.
Get a few blank lines between the previous posters comment and your comment.
Now, type your comment.
Select the previous posters comment that you want indented and highlighed as a quote.
Click the >> button, chose 'Quote'
Viola. Repeat. Watch the video for more . . .