When you start a sentence with a dependent clause, it weakens that sentence.
(Yes, I'm aware of the irony of starting that sentence with a dependent clause. But this is a blog post, not a blurb or story, and nobody's paying me for my blog posts, so it doesn't matter. ;) )
And that's it. Keep that rule in mind, and then edit your lines accordingly. For example, if you're writing a blurb for your book, then you want your sentences to be as strong as possible. You want that blurb to grab the prospective reader by the virtual lapels, give him a good shake, and say, "This book rocks! Click 'buy' already!" You don't want a blurb that says, "Well, you might like this book. Or not. I'm not that confident about it, but please give it a try anyway."
Let's examine one of my own blurbs. Here's the blurb for Clouds of Venus:
Dale Kinmont is a college student in post-catastrophe America. He's lucky; he lives in one of the walled cities for the nation's elite, and life is pleasant. He expects to graduate and find employment in his uncle's company.
Everything changes when he's framed for murder. He's tried, convicted, and sentenced to hard labor in the prison colony on Mercury.
He ends up in Hesperus instead, a flying city that soars eternally through the acidic skies of Venus. His goal now is to find a way to clear his name and return to Earth before Hesperus erupts in civil war. He also must battle the harsh realities of the planet itself. Because if the Hesperans don't kill him, Venus probably will.
Only one of those sentences starts with a dependent clause, and it's the very last one. By the time the prospective reader reaches that last sentence, his or her mind is probably already mostly made up about whether or not to purchase the book. Also, the rules of blurb-writing are different for the last sentence of the blurb, because that sentence is the final "hook" that, ideally, convinces the reader to grab the book. For the rest of the blurb, though, it's best to follow the guidelines. Save the dependent clauses, sentence fragments, snarky puns, and whatever else for that last line.
Now let's re-write that blurb with the other sentences rearranged so that they start with dependent clauses:
In post-catastrophe America, Dale Kinmont is a college student. Living in one of the walled cities for the nation's elite, he considers himself lucky, and life is pleasant. Upon graduating, he expects to find employment in his uncle's company.
When he's framed for murder, everything changes. Tried and convicted, he's sentenced to hard labor in the prison colony on Mercury.
Instead, he ends up in Hesperus, a flying city that soars eternally through the acidic skies of Venus. While there, his goal is to clear his name and return to Earth before Hesperus erupts in civil war. Also, he must battle the harsh realities of the planet itself. Because if the Hesperans don't kill him, Venus probably will.
It's basically the same blurb, but it lacks some of the punch, doesn't it? That's because dependent clauses are sort of "passive-aggressive," or at least more so than independent clauses. The natural position in a sentence for a dependent clause is after the independent clause, not in front of it. When we swap those positions, the sentence feels a little weaselly, as if the writer was trying to hedge his bets by not fully committing to the sentence's meaning. That's because the sentence's meaning is based on the independent clause, but it's only modified by the dependent clause. The difference in the effect of these two methods of sentence construction is subtle, but it's real, especially on a subconscious level, and it can mean the difference between a sale and a non-sale.
Most of the time, both in blurbs and in the text of your story, you'll want to start your sentences with independent clauses. That makes for stronger sentences. Strong sentences are more engaging than weak ones, and reader engagement is crucial, especially in blurbs.
So that's my craft advice for today. Have a look at your blurbs and see if you can re-write your sentences so that they start with independent clauses, not dependent ones. If you do, then I think you'll notice a positive difference. Good luck! :D