Question

All the students think interesting and useful to speak Engli

Answer 1

应该是 it

-------- All the students think IT interesting and useful to speak English.

语法分析：

All the students（主语） think（谓语） IT （形式宾语）interesting and useful （宾语补足语）to speak English（真正宾语）.

所以 it 是形式宾语,在你的句子裏面不能省略.它的存在是为了句子的平衡,避免头重脚轻.