Your high school English teacher probably told you to never precede because with a comma. Maybe they even told you it’s because it’s a subordinating conjunction—it connects an independent clause and a subordinate clause—and subordinating conjunctions don’t typically take a comma.
But sometimes not using a comma can create confusion. Here are a couple examples a reader sent to me via Facebook Messenger recently:
The riot is spreading throughout the country because my brother told me.
If you follow the strict rule of no commas before because, then this sentence would mean the riots didn’t start until the speaker’s brother told them. To avoid confusion, I’d include a comma.
The riot is spreading throughout the country, because my brother told me.
Even so, the sentence is still awkwardly written, but that’s beside the point. Use a comma here to avoid confusion.
Here’s the other example:
I need to go home because it is going to rain unless you want to give me a ride home later.
Leaving a comma out here means that it won’t rain if the person wants to give the speaker a ride. I’d rewrite it as
I need to go home because it is going to rain, unless you want to give me a ride home later.
So, in short, it’s a good practice to not add a comma before because, but don’t become so pedantic about it that it ruins the meaning and flow of your sentences. Your writing should always be as clear as possible.
