Sally closed the window because the weather outside was cold.
the CAUSE is the fact that the room was cold, and the EFFECT is Sally's closing the window. The conjunction because is placed in the correct position here, which is right before the cause. Similarly, in the sentence
Because the weather outside was cold, Sally closed the window.
the conjunction because is correctly placed before the part of the sentence that expresses the cause, even though the subordinate clause because the room was cold is now at the beginning of the sentence. (Note that the first letter of the conjunction is now capitalized.) However, in this sentence:
??The weather outside was cold because Sally closed the window.
even though it is grammatical, it does not make sense because a person's opening or closing a window does not influence the weather.
Second, you should be careful when using commas. Conjunctions such as therefore, consequently, as a result, and for this reason are usually followed by a comma, as in these examples:
The weather was cold; therefore, Sally closed the window.
The weather was cold. Therefore, Sally put on her coat.
The weather was cold. Consequently, Sally put on her scarf.
A blizzard hit the town. As a result, the schools were closed.
The adverbial clause conjunctions since and because are exceptions. These are attached directly at the beginning of CAUSE-sentence without a comma, as in the example above, Because the weather outside was cold, Sally closed the window. The comma here is placed at the end of the subordinate clause.
The coordinating conjunction so is also different from the ones above. This conjunction has a comma before it, as in this sentence:
The weather was warm, so Jim turned on the air conditioner.
However, in formal academic writing, so may not be used at the beginning of a sentence (although you will often see it in informal writing):
The weather was warm. So Jim turned on the air conditioner.
(too informal -- avoid this usage)
