Here's a breakdown:
* When to use a comma:
* If "unless" introduces a dependent clause: If the phrase after "unless" cannot stand alone as a complete sentence, you need a comma.
* Example: "We will go to the beach tomorrow, unless it rains."
* For clarity and emphasis: Sometimes, even if "unless" introduces an independent clause (can stand alone), a comma can help the reader understand the sentence's structure.
* Example: "I'm going to finish this project, unless you have a better idea." (This sentence is grammatically correct without the comma, but the comma helps to emphasize the conditional aspect of the statement.)
* When not to use a comma:
* If "unless" introduces an independent clause (can stand alone):
* Example: "We will go to the beach tomorrow unless it rains."
In short, the presence of a comma before "unless" depends on whether the phrase following "unless" is a dependent or independent clause.
It's always a good idea to read your sentence aloud and see if the pause before "unless" feels natural. If so, a comma might be appropriate.