Why is the hydride ion negative?
And in what conditions can $\ce{H^{−}}$ occur?


In any phase. The hydride ion however doesn't stick around for long in water since it's such a strong base. First, hydrogen isn't that electronegative to start with, so tacking on an extra electron isn't helping with its stability. Second, combination with an hydrogen proton, $\ce{H+}$, is highly entropically favorable because the product, $\ce{H2}$, is a gas. 

The $pK_a$ of $\ce{H2}$ has been estimated to be ~35 so that speaks a lot to the strength of the hydride ion as a base.