Zero line spacing

I just started working on Scrivener. I went through the tutorial, then opened a non-fiction project with sub-heads. Then I created the structure of the book in the binder with chapters, sections, and sub-sections. So far so good.

But when I actually started to type text into the editor, I ran into a atrange problem. When I get to the end of a line the cursor returns to the left margin, but on the same line. If I keep typing, I’m just typing over the text that is already there. The line spacing box (just to the right of the font size box) says 1.0x

If I copy a paragraph, the same thing happens. All of the text is copied on one line and is of course unreadable. I don’t see anything about this in the forum or the manual. How can I fix this? Thanks.