All in the title.

You can comment on someone's profile by clicking on their name, which will lead you to their profile, then you can comment down below (as long as they haven't turned commenting off). As for private messaging, Scratch does not currently have a private messaging feature, as it can cause problems with moderating. So if you want to talk to someone directly, the only way is to comment on their profile or reply to a comment of theirs.

oh ok.