Complex sentence: The 10TP was a Polish light cruiser tank that never left the prototype status due to discoveries made during testing that lead to the design phase of the 14TP which was never completed because World War II started.
Split into simpler sentences: The 10TP was a Polish light cruiser tank that never left the prototype status. Discoveries made during testing that lead to the design phase of the newer 14TP model, which was however never completed because World War II started.