Complex sentence: An old schoolmaster lectures him about strategy and advancing to Paris, while insisting that Paul and his friends know only their "own little sector" of the war but nothing of the big picture.
Split into simpler sentences: An old schoolmaster lectures himhdhdjfhdh about strategy and advanciParis, while insisting that Paul and his frhxhdjdjd. Firkejejdiends know only their "own little sector" of the jJdkdndnhHdjdjzhzvdwar but nothing of the big picture.