* At the beginning of a sentence: "Because it was raining, the picnic was cancelled." (The rain is the reason for the cancellation.)
* In the middle of a sentence: "The picnic was cancelled because it was raining." (Similar meaning, different structure.)
* With a comma (when it starts a subordinate clause): "I stayed home, because I was feeling ill." (A comma separates the main clause from the subordinate clause introduced by "because".) Note that some style guides might suggest avoiding starting sentences with "because," but it's grammatically correct.
* Without a comma (when it's less of a distinct clause): "I'm tired because I worked late." (The reason is more directly attached to the main clause; a comma is less common here but still acceptable depending on style.)
In short, use "because" to show cause and effect. The clause following "because" explains *why* something happened or is true.