Ocean trenches....? GCSE Geography

Can someone explain the formation of ocean trenches please? All I know is that it happens at a destructive plate margin......?

the oceanic plate moves towards continental playe due to convection currents in the mantle.
The denser oceanic plate sinks beneath the continental plate.
As the denser oceanic plate sinks beneath the continental plate it starts to melt in the subduction zone
oceanic trenches are formed at the subduction zone

I hope that helps

Hi my teacher gave me this paragraph of how a ocean trench forms: On a destructive margin, two plates move together. The denser oceanic plate is forced underneath leaving a large gap or trench which is but deep and runs the length of the plate boundary. This is called an ocean trench and it occurs at the edge of the plates in the subduction zone.
Hope it helps