您好，很高兴为您解答如下：
是fall in love 吧
这句话，不能用现在完成时态，因为“有一天”是一个过去的时间点，而非时间段。
One day, we fell in love with each other.
去掉“有一天“，则可以：
We have fallen in love with each other.
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one day,we have been falling in love
One day, we fell in love
We have falling in love one day.
One day,we are falling in love.
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