I know comienza is correct but I can’t think of any reason why the subjunctive would be correct
I don’t know the rules, I just know it’s like this. You can use both. Anyway comience sounds a bit better
thanks, I can’t argue with a native speaker. I suspect it has to do with the idea that the sentence is hypothetical or unknown… we aren’t talking about a specific sentence but a theoretical one. Does that sound correct to you?
Yes, it does.