Once that has happened it will be very difficult to reverse.
一般这种情况发生，就会很难扭转。
It happened once in the middle of an interview.
这以前在一次采访中间曾发生过。
It happened once in the observable universe, and we are it.
它只在这个可观测的宇宙里发生了一次，我们赶上了。
Once it so happened that a rat fell into a jar.
一次，有一只老鼠忽然掉人瓮中。
Once it so happened that a rat fell into a jar.
一次，有一只老鼠忽然掉人瓮中。
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