It is ...,通常放在句首,it代表后面要說的事.
that is ...,通常放在句子中作為從句；當(dāng)然也可以放句首,that特指某事；還有習(xí)慣用法如：That is to say...也就是說...
沒有It is to difficult,That is to difficult的說法.
1）It is too difficult ...表示...太難了.如：
It is too difficult for me.
It is too difficult to do it.
2）It is difficult to do...表示做某事有困難.如：
It is difficult to finish my homework to night.
3）I don't try something that is too difficult.
最后建議,it is和that is用法相當(dāng)廣泛,最好的辦法是通常大量閱讀提高語感,熟悉其用法.