That creates situations like the one where a student turned in a paper with this
The landlord refused to respond to inquiries. Because he said he needed to talk to his lawyer.
When told the phrase beginning with “Because” was a sentence fragment, the student objected:
But you said it was OK to start sentences with conjunctions. “Because” is a conjunction. And if you take off “Because,” it’s a whole sentence. Why is it wrong?
Thus arose the lesson that not all conjunctions are created equal, and that they sometimes cannot begin what look to be sentences.
Conjunctions, like so many other parts of speech, have multiple roles. There are “coordinating conjunctions,” “correlative conjunctions,” and “subordinating conjunctions.”
“Coordinating conjunctions” are the ones that are fine to use at the beginnings of sentences. They merely link two similar constructs, be they words (“blue or white”), independent clauses (“They’re not real, but they sure look like they are”), or sentences (“She wasn’t happy. And she wanted to know why.”). Most usage authorities say there are only seven “coordinating conjunctions,” known by the acronym “fanboys”: “for,” “and,” “nor,” “but,” “or,” “yet,” and “so.” Those are the ones you can use at the beginnings of complete sentences with impunity.
We’ve talked a bit about correlative conjunctions, like “either … or” and “not only … but also,” which connect balanced sentence parts. You can start sentences with those as well.
Now we have to focus on the “subordinating conjunctions.” Yes, it sounds a lot like grammar. But it’s not as hard as it sounds.
A “subordinating conjunction,” like all the other conjunctions, also connects two parts, but it does so by making one part dependent on the other. In “I hate pistachio ice cream because it tastes like chemicals,” the conjunction “because” introduces the reason the pour soul hates pistachio ice cream. Without the ice cream, the phrase “it tastes like chemicals” has no reason to exist: It’s part of a dependent clause, even though it is also a complete sentence without the conjunction. By definition, a dependent clause has to be part of another sentence or it becomes a sentence fragment. So it would be improper to separate them into two sentences: “I hate pistachio ice cream. Because it tastes like chemicals.”
A “subordinating conjunction” effectively creates a cause and effect, explaining the why, when, where, how, or conditions applying to the main clause. In addition to “because,” common “subordinating conjunctions” include “after,” “if,” “once,” “since,” and, of course, “when” and “where.”
You can use “because” and other “subordinating conjunctions” at the beginning of sentences that include the clause they depend upon, as in “Because it tastes like chemicals, I hate pistachio ice cream.” And you can also use a “subordinating junction” at the beginning of a sentence fragment in less formal writing, for emphasis: “Because I said so, that’s why.” Be sparing of those, though.
To avoid creating a sentence fragment with a “subordinating conjunction,” watch out for dependency: In our student’s answer, the reason the landlord wanted to talk to his lawyer was dependent on knowing that he refused to answer questions. It would have been perfectly fine as one sentence:
The landlord refused to respond to inquiries, because he said he needed to talk to his lawyer.
The Oxford Dictionaries blog has a great explanation of the differences, in case you want more. Because grammar counts.