The film ____we saw the day before yesterday is very interesting.

that
因为有一个 the day before yesterday

反正因为有这个东西，所以要用一个特定的代词，我有点忘了，嘿嘿

in which

因为film是物，所以只能用which或者that the day before yesterday 中文意思是前天 是过去得过去 所以只能用that

that

that后面引导后置定语
那么就翻译为我们前天看的电影很有趣