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。