What is it with me and run on sentences?!
Hatless makes a good point.
Although run-on sentences aren't always bad.
The main reason they are generally taught as a no-no is that they can be a little overwhelming and less clear than shorter sentences.
And additionally because generally they contain multiple major ideas.
They can be good when you have a lot of relatively unimportant things you still want to communicate and only a small amount of important ones. You can put the unimportant stuff in the the middle of the run-ons. And put the import stuff at the end and beginning of them. Or in shorter sentences which will stand out in contrast to the longer ones.
However in dialogue or character thoughts they can be used to express a smart character who likes to talk a lot. Since run-on sentences would either take a while to say or would involve speaking very quickly. Really short micro sentences are better for actiony type fast paced scenes.
It also can be great for someone who is sort of mentally spiraling, unfocused, confused, really excited etc.
Particularly in thought-speak/writing but also sometimes in dialogue.
If you don't use too many run-on sentences then they can really pop and draw attention to themselves in a context of midlength or shorter sentences.
Long sentences in general.(Not necesarily run-ons)
'
John walked tiredly down the windy road, through the tall, breezy cornfield, and into the gigantic hardware store.(Long but not a run-on) He wandered the Automotive section till he grabbed a gas can with ragged breath. (Arguably a run-on. this could easily be two sentences. But has only one action technically with how it is written.) He sighed. He didn't enjoy looking forward to the trek back to his car.'
But using a bunch of clauses that share the same action and that come after one another in time can be a great way to imply the passage of time. Doing that is fairly clear and also gives you long sentences. More generally making it so the ending of one longer sentence and the beginning of the next share the same subject makes things flow better and are clearer. Sometimes that isn't feasible though. Sharing the same subject as the last sentences is also fairly clear but often not as much. Unless it's a simple subject and the next sentence begins with a pronoun.
Setting sentences on their own makes things seem faster.
It draws focus.
It darts down the page.
It can't communicate as much information. But, makes sure that important things are highly emphasized. Using a short sentence set on it's own line after a long paragraph with long sentences is probably one of the strongest forms of directing focus. Although how wide of a writing area you have effects how much that effect manifest. A narrower writing area makes it easier to make a paragraph feel long when you want it to, But if it's too narrow then any paragraph with more than 4-5 sentences of decent length starts feeling long.
When you have a wide area, like this one, you can put multiple lengthy sentences on the same line and still feel relatively short. Or just one or two long ones.
It's still not as punchy as just one medium length sentence on it's own.
Much less with short ones.
But trying to look for run-on sentences in your writing is a good idea, to play with at least. The don't always have to be removed. But it is valuable to think of alternative ways of saying the same thing. Sometimes they are more clear, Or come together to create an effect you want more powerfully.
It isn't just about what you say or write, but how you write it out, and communicate that content.
I learned most of this from a Rhetorical grammar book I read for a class.
Those types of books are gold mines for micro scale writing. And making grammar a tool for you to manipulate to get the effect you want rather than just a set of words.
I read a couple of those and they essentially give you a whole bunch of techniques you can use to transform how you are saying something or manipulate emphasis within a sentence, paragraph, page, or even a work.
And how those interact with clarity.
As well as things like the feeling of the words, emotion of the moment, and pacing and flow of the scene.
Creative Grammar books are also useful and fun but not quite as much as rhetorical ones.
Now you can get some style handbooks if you want to make sure your writing is very clear.
And they will give you a curated set of rules based on the authors opinions about writing.
And it is definitely worth looking at some of those, But they aren't as fun. And also are not easy to practice.
And it mostly gives checklists to use when editing.
Editing checklists are valuable. Although, in my view editing in a creative manner is often a way to unlock ways to word things that are amazing and not just good.
Actively thinking about how well your writing works and what it is trying to accomplish and communicate at every level can give you a further level of control.
From the level of how clauses are ordered and constructed inside a sentence. (Or even words.) Sentences as a whole, and how they fit into paragraphs.
And how paragraphs contrast and flow into one another to set the pacing of the scene and create the emotional or characterization effect you want.
In some cases curating "word list" can also be an amazing way to help give a particular feeling to a story. I usually don't bother with that until later on it editing, because it isn't easy. But whenever I have actively designed a word choice for a space I have really liked how precise it made the feeling of the story.
Lot's of things can effect how a word feels, You can go for alliteration. (Or shared syllables or letters at the beginning.) Or rhymes for obvious stuff.
But more generally going for more words that use particular consonants or types of ones can change the feel, similarly with vowels.
Or for shorter vs. longer words.
(Or even root language, Using Saxon root words has a different feel to Latin, french, or Greek.)
If you have a character with a unique profession then looking up words specific to the trade of the protagonist can be a great way to subtly indicate their knowledge and make it feel a bit more authentic. Or Jargon and such.
If they are a logger you can look up specialized logging terms and use them.
Or if they are a carpenter you can try to adjust how you describe things to use carpentry terms and emphasize what that character would be likely to notice about the environment.|
More generally using more specific words can often allow you to communicate more with less words or sentences.
Going back to run-ons. Sometimes they are actually more clear than any of the other alternatives you have been able to think of.
If you have a paragraph made out of a lot of short sentences which don't say much, then 2-4 longer sentences very well could be more clear.
Although, doing that can have an effective cadence which can make up for it being wordier.
But writing out some paragraphs using only short sentences.
(Or sentences with a certain rough length.) Is a great exercise.
And then once you do that trying to condense it down into 1-3 long sentences. Or even full on run-ons.
Often when trying to condense you can actually find ways to express or imply more info than what you originally used more words for.
(In addition to cutting a lot of filler words that only serve a grammatical function rather than a content one.)
Doing the reverse is also a great excercise. Take a run-on or a paragraph made from 2-3 run-ons and try to split them up into shorter sentences while not loosing any elements of meaning. (Or even adding some.)
Having too many or too few words that only serve a grammatical function generally tends to make things less clear.
(Which is another thing which can make run-ons less clear.)
When things have a good enough flow you can even omit words and still have things be very clear.
But if it is poor and you are missing something you just make it confusing. And on the other hand, a lot of grammatical words can bog down a sentence since the actual meaning is buried under words for showing relationships between concepts. (That is technically a seperate issue from run-ons. But is a common reason why they are unclear.)
But ultimately you want to be in control of how you use grammar, rather than letting poor grammer or even just a strict set of rules control your writing.
And more generally looking for exercises that let you practice tools within a small area of writing, or that make you put the tools you know to the test in a small context are a great way to improve your ability to wordsmith.
Trying to communicate a very specific feeling in a passage without actually saying what it is can be great practice too.
Can you make the prose itself feel angry?
What about sad narration?
Or happy?
Pretentious?
Generally very vague and very specific feelings are the hardest to communicate indirectly in writing.
Like the bittersweet feeling of morning someone who died a long time ago while also feeling happy to reminesce about the good memories. Perhaps while looking through a photo album.
You can do it a lot easier by relying on cliches or specific actions and events that are already associated with the feeling by the reader.
But what about trying to capture that feeling during a fight scene?
Anyways, I hope this mini-essay on writing and grammar is useful to you.