This is an entire course, very helpful
This sentence has five words. Here are five more words. Five-word sentences are fine. But several together become monotonous. Listen to what is happening. The writing is getting boring. The sound of it drones. It's like a stuck record. The ear demands some variety.
Now listen. I vary the sentence length, and I create music. Music. The writing sings. It has a pleasant rhythm, a lilt, a harmony. I use short sentences. And I use sentences of medium length. And sometimes when I am certain the reader is rested, I will engage him with a sentence of considerable length, a sentence that burns with energy and builds with all the impetus of a crescendo, the roll of the drums, the crash of the cymbals--sounds that say listen to this, it is important.
It is more difficult to make a long sentence clear than it is to make a short one clear. If, then, you are a poor writer and want to make sure that youngsters understand you, stick to short sentences. Unfortunately, a long series of short sentences, like a long stretch of writing with no “hard” words, is irritating to anyone intelligent, young or old. A youngster is particularly offended because he thinks (sometimes with justice) that the writer thinks that because the youngster is young, he is therefore stupid. The book is at once discarded. (This is called “writing down,” by the way, something I try never to do.)
The trick is to write clearly. If you write clearly enough, a long sentence will hold no terrors. If you hit the proper mix of long and short, and hard and easy, and make everything clear, then, believe me, the youngster will have no trouble. Of course, he has to be an intelligent youngster, but there are a larger percentage of those than of intelligent oldsters, for life hasn’t had a chance yet to dull the youngsters’ wits.