Water over the dam
«Eso es historia /
Ya lo pasado, pasado
Things that belong to the past cannot possibly be changed. We can think about them, learn from them, reflect about them, but never change them.
The idea behind this expression is simple. When water flows, as in a river, it cannot go back just by a mere wish. We use this expression to refer to thing that have an irreversible outcome. Even so, we can use the third conditional to think about them, and to learn a moral from them, but we cannot go back to the past and change the situation. A similar expression to this one is, «water under the bridge.»
Mom: Ok, hun. You are a teenager, you are single, you don´t have a fiancé and wow, we know you are pregnant. What are we going to do?
Girl: Oh, mom! I wish I could go back in time and refused to have gone to that lousy party.
Mom: Bla, bla, bla… That is water over the dam. Set your feet on the ground. What are we going to do?