Sometime around second or third grade, most people learn that it’s not ok to start to start a sentence with the word “because.” While I have no desire to cast aspersion on your third-grade teacher, unfortunately that rule is only half true. The main reason is gets taught that way is that seven and eight year-olds have a tendency to write sentences like this:
Because I went to Disneyland last summer!
Because I played hockey with my brother last weekend.
As I hope you can recognize, neither of these is a full sentence; it is unacceptable to begin the sentences such as these with “because.”
Unfortunately, though, a lot of the time no one bothers to teach the other half of the rule three or four years down the line. The truth is that under certain circumstances it’s perfectly fine to start a sentence with “because.”
Here’s when: a clause beginning with “because” (e.g. “because I stayed out late last night) must be followed by a complete sentence.
Correct: Because I stayed out late last night, I fell asleep in math class this morning.
Since the second clause, “I fell asleep in math class this morning,” works as a stand-alone statement, it’s perfectly acceptable to start the entire sentence with “because.”