By hide, do you mean to not be able to see the text until the plus button that appears on some comments is pressed? If so, just press the enter button a few times until your comment has a relatively large amount of white space and you should be good. I'm not a hundred percent sure exactly how many times you should hit enter, but probably around 4-5+ times for good measure.

Hello! I just started exploring my account, I tried to comment on something and I intended to hide some of the text but I think it wasn't successful. Can you please help me? Thanks in advance!