He is expected to be the key witness at the trial.

预计他将成为审判时的关键证人。

He is a key witness in this case.

在这个案件中，他是一个主要见证人。

The mob is planning to rub out the key witness.

暴力团伙正计划干掉重要目击者。

Police: I understand you were the key witness for the robbery.

警官：我了解到你是这起抢劫案的主要目击者。

For reasons of personal safety, one key witness wishes to remain anonymous.

出于个人安全的考虑，一个主要的证人希望不透露姓名。