1. Complex Sentences

2. Plain English
If a sentence begins with a phrase, clause, or transition, you probably need a comma to separate it from the independent clause that follows.
In other words…
Use a comma after an introduction or an opener.

3. What’s it look like?
Dependent clause + , + independent clause + . = complex sentence. or
Opener, sentence.

4. Dependent v. Independent
An independent clause has a noun and a verb and expresses a COMPLETE thought.
You are in eighth grade.
A dependent clause has a noun and a verb and does NOT express a complete thought.
Since you are in eighth grade.

5. AAAWWUBBIS!
Certain words can signal writers that a comma may be needed after the opener.
When an AAAWWUBBIS words is the first word of a sentence, a comma will be needed.
A.K.A… subordinate conjunctions

6. AAAWWUBBIS Words As When Before Although Unless If After Because Since
While

7. A few examples…
If you can’t annoy somebody, there is little point in writing. ~Kingsley Amis, The King’s English
When summer comes to the North Woods, time slows down. ~Jennifer Donnelly, A Northern Light

8. Complex sentences CAN be combined with other sentence structures.
When I saw the woman, she reminded me of a bird. Though her hair was white with age, she walked with small, quick, lively steps. ~ Laurence Yep, The Star Fisher