Complex sentence: Only after the fall of Tarnovo in 1393 his policy became more active and eventually joined the crusade of the Hungarian king Sigismund but after the disaster in the battle of Nicopolis in 1396 the Ottomans marched to Vidin and seized it.
Split into simpler sentences: Only after the fall of Tarnovo in 1393 did his policy become more active and he eventually joined the crusade of the Hungarian king Sigismund. However, after the disaster in the battle of Nicopolis in 1396, the Ottomans marched to Vidin and seized it.