She regarded me with some confusion. "What do you mean, vary?"
"You know, make them different lengths, so you don't have two short sentences in a row."
She said slowly, "But what if I need two short sentences in a row? I'm not going to make a long sentence just because the one before is shorter."
"But what if too many short sentences in a row sounds too juvenile?"
"It would only sound juvenile if I wanted to sound juvenile. If I wanted to sound terse, it would sound terse. It's not the length of the sentences that I care about, but what they convey, and if what I wanted to convey went best in a bunch of short sentences, that's what I'd use. If not, I'd use sentences that do what I want."
I didn't bother to ask her about intro participial phrases, because I figured I knew what she'd say. If she needed a sentence to start that way, she'd start it that way. Otherwise not.
What's the point here? It's the meaning that matters. It's the effect that matters. Sentences should be designed to convey some meaning, and whatever length or opening creates that meaning, that's what writers should start with. Now of course, if the sound is wrong, if the sound of the sentences together detracts from the meaning (which is greater, of course, than just the surface meaning of the words in sequence), the writer should explore why and how to fix that-- if one sentence needs another beat, or if starting with a placement preposition ("In France,") would set the stage more efficiently. But the hard part is writing sentences that mean what you want them to mean, and putting them together in an order that adds even further meaning, and adding the connectives and keywords that increase the coherence.
Sentence variety is a minor consideration, if that. If the paragraph or passage sounds wrong, sounds repetitive, if the information is redundant, those are all reasons to revise sentences. And the lack of variety might be a symptom of one of those. But it's just a symptom-- lack of variety in sentences is not, in itself, a problem.
In fact, if you follow a mental set of rules (don't begin two sentences with the same word... don't have the same opening construction more than twice in a paragraph), you will very likely be depriving yourself of some important tools to make your prose stronger-- musical and meaningful.
Just keep that in mind-- if you want more poetic prose, read poetry to discover the useful poetic devices. If you want more musical prose, listen to music to get a sense of the musical devices.
And you know one thing both poetry and music have in common-- repetition. Yep. Repetition of words, phrases, and constructions.
Here's an example (from the immortal Smokey):
I don't care what they think about me
I don't care what they say
I don't care what they think if you're leaving
I'm gonna beg you to stay
I don't care if they start to avoid me
I don't care what they do
I don't care about anything else
But being with you being with you.
Now that "I don't care" repetition isn't just musically appealing (though it is). The accumulation of "I don't care" makes each stanza sound defiant, and more than that, establishes the stakes here-- the singer is choosing his girl over everyone else. He's telling us with the "I don't cares" what he is being warned, what will happen if he makes this choice.
Now there's art to this-- you see in both stanzas three "I don't cares" and then a capping line (one that, btw, finishes the last "I don't care," which adds to both the forward propulsion and the coherence of the stanza). That is, the identical sentence structure of the two stanzas (which is very common in popular music) connects the two thoughts, and also then highlights the distinctive final lines.
One thing repetition does is focus the attention on the non-repetition. You pay more attention to the change because you're used to the repetition. So, in fact, a repetition of phrasing or structure BEFORE an important thought will subtly let the reader know that it's important. (So again, if you want to hide a clue, don't put it in the "important" position-- put it in the middle and make the structure and/or phrasing repetitive-- the reader might "elide" over it and not give it too much attention.)
Here's a poetic example. TS Eliot, a master craftsman (oh, yeah, good thinker and all that stuff too), often used repetition at the beginning of stanzas to unify the stanza or connect it with other similar stanzas:
Between the idea
And the reality
Between the motion
And the act
Falls the Shadow
For Thine is the Kingdom
Between the conception
And the creation
Between the emotion
And the response
Falls the Shadow
Life is very long
Between the desire
And the spasm
Between the potency
And the existence
Between the essence
And the descent
Falls the Shadow
For Thine is the Kingdom
(This is the poem that ends with "This is the way
the world will end, not with a bang but a whimper."
It always reminds me of a friend's brother, whose
ex-girlfriend broke up with him to start dating
a more "suitable" young fella, one who worked
in a bank. The brother said ruefully, "And that's
the way the world will end, not with a whim
but a banker."
And I ask you-- which guy would you choose?
The one who works for the bank? Or the one
who can -- just like that-- make a twist on Eliot?)
So every stanza (this is, btw, near the end)
starts with a "Between (this) and (that)"
and ends with "falls the shadow".
Is the repetition a problem? No, of course
not-- it's the point.
By repeating that formulation, the poet focuses
on the connection and separation of the pairs,
and the reader has to think, "What is
the shadow between the idea and reality?"
And the precision of the repetition of the stanzas
contrasts with the sudden insertion of those comments
("For Thine is the Kingdom").
Yeah, yeah, that's poetry. But prose can sing too,
and you can use that "music" to draw attention
to a conclusion, a striking metaphor, or a theme.
I'm not trying to get you to be repetitive. Rather
I'm trying to refocus your thoughts onto what sentences are:
1) Units of meaning, and
2) Parts of bigger units of meaning (paragraphs and passages).
What creates meaning, what creates the effect you want,
in sequence and in combination-- That is a good sentence.
Paying too much attention to non-meaning factors (like making
the opening sound different) can actually detract from
your ability to give the reader the experience
you mean them to have.
Get the meaning right first. Know what that is. Know it
inside you before you write and before you revise.
(It might help to free-write here before you start.)
Then let your desire to communicate this to the reader
guide you to the best preliminary construction. Then check
on the big things-- is this in the right point in the
passage? Do you need some kind of transition to connect
it to what comes before? Is the sentence clear so the
reader can get your meaning (or however much of it you
want to convey right now)? Is there some poetic or
thematic thing you want to echo in here?
(And of course, if you're writing in character voice,
are there any words or formulations that don't sound
like that character?)
Get that stuff right, and then I don't think the
construction will worry you, because the sentence will
be meaningful and interesting and go well with the
rest of the paragraph.