it was that…Where English uses phrases like "the truth was" or a knowing tone of voice to reveal the real reason behind something in the past, Japanese adds のだった (no datta) to the end of a sentence. It signals that you are giving the background explanation for a situation or sharing a sudden realization about how things really were. For example, 彼は疲れていたのだった (kare wa tsukarete ita no datta) means "The thing was, he was tired." In casual conversation, it is often shortened to んだった (n datta), as in 今日だったんだった (kyou datta n datta), meaning "Oh right, it was today!"