“nail down”可别理解成“指甲掉了”，真实含义时弄清；确定；决定。

【例句】
It would be useful if you could nail down the source of this tension.
如果你能弄清这种紧张情绪的根源，会是有用的。

They nailed him down to a specific time and place.
他们迫使他确定了时间和地点。