6.  It / This / That + the first(second , third …) time + that-clause

这个句型表示截止到说话时为止的某人的一种经历，关键是注意time前有序数词，主句是 is 时 ，从句要用现在完成时；如果主句用一般过去时was ，则从句须相应地用过去完成时。例如：

This is the first time (that) these Europeans have visited the Great Wall .

It was the fifth time (that) I had paid a friendly visit to Africa .

二、It代替从句作形式主语的常见句型