...oh yes, long & short sentences!
Obviously, I'm not yearning for a time machine so I can seize Dickens by the collar & say 'Now see here Charlie boy. More SHORT sentences, right?'
But if - as this afternoon - I am teaching a year 8 group of significantly below average prior attainment year 8s - then it is helpful to them if I take a piece of their writing & show how they can punctuate it for clarity & effect.
So I'll often get them to 'just write, don't worry about spelling, punctuation or paragraphs'.
This afternoon we were writing short descriptive pieces called 'Trapped'. I displayed one of the better examples (some effective similes & nicely judged use of onomatopoeia) on the whiteboard, & got one of the most able students to punctuate it. Then we read it round the class, changing reader at each punctuation mark, & discussed whether we could make the pace more effective with better punctuation.
By the end of the lesson, they had all absorbed that they could make their writing more powerful by using short, sharp sentences for punchy action, long complex sentences for building atmosphere, & long sentences without any punctuation at all even slightly whatsoever to create a breathless panicky feel.
Again, it's mechanical, but it's not something that natural writers don't do - they just do it far less consciously.
& again, if you come from a bookish home you absorb this sort of thing as a matter of course. For students who don't, actually lifting the bonnet of writing & having a good rootle round the engine is the only way to do it.