Read the sentence. unfortunately we had not brought our reusable water bottles and had to buy bottled water. after which word should a comma be added?

The answer is after "Unfortunately". The sentence should read, "Unfortunately, we had not brought our reusable water bottles and had to buy bottled water." I hope this helps!

There has to be a comma after an introductory adverbial element. In this sentence, 'unfortunately' is the adverbial element. Therefore, a comma would have to be placed after 'unfortunately.'