Question: When did Li Chengqian's mother die?
Sentence (from "Li Chengqian"): The plot was discovered, and he was deposed and reduced to commoner rank, but Emperor Taizong, believing that Li Tai to be responsible for Li Chengqian's downfall, appointed yet another son, Li Zhi (also by Empress Zhangsun), crown prince instead.
Relevant evidence: No