Words that can start a dependent clause, and therefore a complex sentence, are called subordinating conjunctions. Some common subordinating conjunctions include:
* Time: after, before, since, until, when, while
* Cause/Effect: because, since, so that
* Contrast/Concession: although, even though, though, whereas, while
* Condition: if, unless, provided that
* Relative Pronouns: that, which, who, whom, whose
Here are some examples of how these words start complex sentences:
* After the rain stopped, the sun came out.
* Because I was hungry, I ate a sandwich.
* Although it was cold, I went for a walk.
* If you study hard, you will do well on the test.
* The book that I read was very interesting.
It's important to remember that not every sentence starting with these words is automatically complex. Sometimes they can start an independent clause in a compound sentence. For example, "Although it was cold, I went for a walk, and I enjoyed it."