Language transfer Forward transfer Backward transfer The U-shape curve Principal branching direction PBD Principle of temporal sequence PSE Crosslinguistic learner performance comparison Contrastive research Metalinguistic and crosslinguistic awareness 語言遷移 正向遷移 逆向遷移 U型曲線 主要分枝方向 時間順序原則 跨語言學習者比較 對比研究 後設覺知與跨語言覺知