I couldn't tell youNOT KNOWused to tell someone that you do not know the answer to their question 我不清楚，我没法说 ‘How much would a rail ticket cost?’ ‘I couldn’t tell you; I always drive.’ “一张火车票要多少钱？”“不知道，我一直自己开车。”
I couldn’t tell you• "Is it going to rain tomorrow?" "I couldn't tell you."
→ I couldn’t tell you at tell(SPOKEN SECT)