Years ago, I came across an obscure outlining tool geared towards the K12 space, which, if I remember correctly, offered a unique feature: It included a granular outlining view where each sentence of a paragraph appeared as a separate bullet on a separate line. So, you could drill down to this sentence-by-sentence level, or go back to the normal outlining level where each paragraph is a bullet. (In other words, periods were the delimiter. Of course that could sometimes create problems, like with abbreviations, but it was still useful.)

Unfortunately, I don't seem to remember what the tool was called. Does anyone know of any tools that have this ability to switch back and forth between integrated paragraphs and sentence-by-sentence view?

This most likely isn't the app you're thinking of, but on the off chance it is... Inspiration was (maybe still is) an outliner aimed at K-12. It had the feature of being able to switch between outline and diagram view. I don't think it had the feature of creating a new line with each period, however.

Steve

Thanks, Steve. That's a great point about Inspiration, which continues to be a useful tool.

In the meantime, I found one way to achieve something very close to what I was looking for: In Visual Studio Code, with the "Markdown All in One" extension enabled and indentation-based folding enabled, create a Markdown document. A simultaneous preview window can be opened. Paragraphs can be written with a new line for each sentence (and with sentences indented/outlined as desired), but in the preview window they will appear as normal paragraphs.

It sounds like you're looking for semantic line breaks. They are described here: https://sembr.org/ and here: https://rhodesmill.org/brandon/2012/one-sentence-per-line/

Several markup languagues support them, including Markdown. Here's how some markdown editors handle them:

In Typora, if you write your text using semantic line breaks in source mode, they disappear in live preview mode provided you have the 'preserve single line break' option turned off.

You type one sentence per line in Notepad++ and use the Markdownviewer++ plugin, which can show both texts side by side.

In Obsidian, if you have 'strict line breaks' turned on, you can type with semantic line breaks in live preview mode and have them disappear in reading mode.

Writemonkey 3 has a toggle option (Ctrl+F8) that will switch between one sentence per line and normal text. This one has the benefit that you type normally, whereas in the previous three you have to put single line breaks between sentences.

If you have existing text that you want to break up, you can paste it in here: https://www.onesentenceperline.com/

Awesome, Cyganet! Thanks for filling me in on the proper terminology and providing these resources. Very interesting. Have you tried granular outlining with these? Here's how an outlined paragraph looks in Visual Studio Code:

https://share.cleanshot.com/xjHvSltB

I learned the one sentence per line technique from a writing course, as a method to encourage variable sentence lengths. I haven't tried using it for outlining, because I typically outline ideas with nested bullets (even in plain text notes, emails, and so on).
Your question triggered a memory of having seen this feature somewhere, and indeed it was WriteMonkey 3 that had it.
Then I found out that the Markdown spec ignored single line breaks, and now I know why because that never made sense to me before. The longer I think about it, the more programs there are that support semantic line breaks. My online writing course notes, for example, are in ConnectedText which, you guessed it, ignore single line breaks in source mode when creating the reading mode page.

All very interesting, Cyganet. And yes, I'm remembering that now from ConnectedText. As far as having the best of both worlds, I'm now realizing that with Microsoft Word, it's possible to edit in both semantic-line-break mode (with outlining features) and regular paragraph mode at the same time, by making the line breaks be hidden text (an awesome feature in Microsoft Word -- the easiest way is to create a special linked style for which text is hidden). Here's a screenshot, with Outline view on the left and Draft view on the right:

https://share.cleanshot.com/cg6qcF1Z

Lucas wrote:

... I'm now
realizing that with Microsoft Word, it's possible to edit in both
semantic-line-break mode (with outlining features) and regular paragraph
mode at the same time, by making the line breaks be hidden text (an
awesome feature in Microsoft Word -- the easiest way is to create a
special linked style for which text is hidden).

How do you do that?

Do you need to add a space character after each full stop? In your screenshot the spaces are missing.

with Outline view on the left and Draft view on the right:

@Franz Grieser:

True, I noticed after posting that spaces were missing. My mistake. Normally, I use this technique for writing, so I would incorporate the outlining aspect from the beginning, but to break up an existing text, as I did in the screenshot, the steps would be something like this:

1) Create a linked style for which text is hidden ("Hidden" is available in the font menu). Let's call the new style "MyHiddenStyle".
2) For the text you want to break up, use find/replace to replace all original paragraph breaks with something temporary, say, "myparagraphbreak".
3) Use find/replace to replace every ". " [period plus space] with ". ^p" [period plus a paragraph break plus a space].
4) Use find/replace to replace every paragraph break with a paragraph break that uses "MyHiddenStyle". (Find/replace includes Style choices.)
5) To restore the original paragraph breaks, use find/replace to replace "myparagraphbreak" with "^p".
6) In my example, I also added paragraph headings manually (e.g., "Paragraph 1") and put them in MyHiddenStyle. And I used Outline View to organize the lines hierarchically.
7) Finally, there are various ways to configure what is displayed in each window. In the screenshot, I had used Mac, so I would have to double-check how it is on Windows. But on Mac, for each window, it seems to be possible to use Settings to independently choose which hidden characters are shown. So, for the Outline View window, I went to View | Show Non-Printing Characters and selected "Hidden text". Whereas for the Draft View window, I didn't have any hidden characters displaying, and thus the hidden text is not visible.

(Of course, in my posted screenshot, I messed up the find/replace steps and accidentally removed the spaces between sentences.)

Thanks Lucas.

I was wondering what you meant by "linked style". I usually don't work in Word but most of my writer clients do.
I find the "one sentence per paragraph" trick helpful to check sentence length and rhythm (not for writing but in the editing stage).

@Franz, that makes sense. The other resources Cyganet posted could also be useful for using at the editing stage. A "linked style" in Word is a style that functions both as a paragraph style and a character style. When you create a style, you can choose "linked style" as the style type.