Preview - leave your comment via the feed preview:
Scroll down the feed and find the post that you want to comment on. At the bottom of the post, you can see if others have already commented on that post, review their comments, or leave your own comment.
Simply type your comment, you can also attach a file or photo, and then click 'Post'.
FullView- Open the post on full view and leave your comment:
By clicking on the post, it will open it in a full view window. On the full view, you can read the full post/article (in case it's too long to display on the preview), check if others commented on it, and of course, leave your own comment.