Let it be.随他去吧。

Let nature take its course. 听其自然/顺其自然。

Keep it casual. 顺其自然。

What must be, must be=What(ever) will be, will be. 万事不必苛求，顺其自然。

Let something slide. 放任某事/任其自然。

Leave it as it is/they are.听其自然。

Go with it 跟着感觉走

Roll with the punches 逆来顺受,从容应付困难。