What is a sentence opener?

A sentence opener is simply the first part of a sentence. Most writers default to starting with the subject (“The dog…”, “She…”). Varying the opener — beginning with an adverb (“Slowly, …”), a subordinate clause (“Because the gate was open, …”), or an -ing phrase (“Shivering, …”) — adds rhythm and lets you control emphasis.

It's a small change with a big effect: a paragraph where every sentence starts differently reads far more fluently than one that doesn't.