At the end of a paragraph I hit return ONCE. (I check to make sure, using format - show invisibles, and yes, there is one return.) Yet, even though I my entire document is single spaced, Scrivener inserts a double spaced line break that I can’t seem to get rid of. I tried selecting the whole thing again, just to make sure, clicking on single space, but again everything is single-spaced EXCEPT the line after the return. I tried typing a single word and hitting return – same thing, double-spaced.
From your description it sounds like you have Paragraph Spacing set to greater than zero (either before or after). This is used to insert extra lines between paragraphs without having to press Return twice (which you don’t want…).
Select one of the paragraphs and make sure that the Formatting Bar is on (toggle cmd-shift-r), then choose the drop down list with the line-spacing showing – it’s towards the right edge and it will show something like 1.0 / 1.1 etc. At the bottom of that list, choose ‘Other’ and see what you have in the Paragraph Spacing boxes. Set them both to zero and see what the effect it.
If that works, then go to preferences and in either the editing / formatting section (can’t remember which off hand – it’s the one with the dummy text editor which also has a formatting bar) make the identical change so that all new documents will follow the same end of paragraph spacing in future.
Then highlight all the documents in the binder that you want to change and go to Documents > Convert > Convert formatting to default (the wording may be slightly different, but it’s obvious) and follow the instructions. This is (obviously) get rid of the extra spaces for existing documents.