話し手の判断、評価
話し手の判断を語る。

Books are fun to read.
It is fun to read to read books. it 形式主語

Your car is too small to carry all of us.
この文の時には、it は、使えない。
I am sorry to say this.

These documents are ready to sign.

It is possible for me to go with her.


スキットから
I am too busy to talk to you. 忙しすぎて、話すひまなんかないよ。
It is impossible for me to follow you.


まとめ
it 形式主語が使えない表現
I am sorry to hear that.