Complex sentence: Susan tries to look over a wall, but slips and twists her ankle, at the same time that the bridge under which the TARDIS has materialised collapses, burying the TARDIS and blocking the travellers' access to it.
Split into simpler sentences: Susan tries to look over a wall, but slips and twists her ankle. At the same time the bridge under which the TARDIS has materialised collapses, burying the TARDIS and blocking the travellers' access to it.