It is ...,通常放在句首,it代表后面要说的事.
that is ...,通常放在句子中作为
从句;当然也可以放句首,that特指某事;还有习惯用法如:That is to say...也就是说...
没有It is to difficult,That is to difficult的说法.
1)It is too difficult ...表示...太难了.如:
It is too difficult for me.
It is too difficult to do it.
2)It is difficult to do...表示做某事有困难.如:
It is difficult to finish my homework to night.
3)I don't try something that is too difficult.
最后建议,it is和that is用法相当广泛,最好的办法是通常大量阅读提高语感,熟悉其用法.