An ocean trench forms when?

Forms by subduction, when two or more of earths tectonic plates converge. The denser plate is pushed under the lighter plate.

Trenches are formed by subduction, which is a process when two or more of Earth's tectonic plates meet and the older, heavier plate is pushed beneath the lighter plate deep beneath the mantle, causing the seafloor and outermost crust to bend and form a steep, v-shaped depression.