Editing sentences in other peoples' lessons
Changes you made apply to all users. All Premium users can edit lessons.
Yes, you can edit content that has been shared by others, your edits will be saved and are visible to anyone who takes the lesson after you. So please exercise caution. If possible, try contacting the user who shared this material to discuss your edits, I at least would appreciate that. This feature is probably best used for editing time stamps which should bee harmless in general. Some courses have been "locked" to prevent editing (and vandalism), for example the mini stories.
Maybe it's an error. This is not good for your own lessons.