Example:
* The weather was terrible, therefore we stayed home.
However, there are exceptions:
* When "therefore" is used at the beginning of a sentence, it is followed by a comma.
* Therefore, we decided to postpone the trip.
* If "therefore" is used in a very short, simple sentence, it may not require a comma.
* The weather was terrible therefore we stayed home. (This sentence is grammatically correct, but using a comma would be more common.)
It's always best to use your best judgment and strive for clarity in your writing.