1. A truism is a statement that is repeated so often that it has become boring.

2. A truism: a statement that is repeated so often that has become boring.

1句是词典原句，请问2句正确与否，正确的话为什么so...that...从句会省掉主语it？

类似的：

3.The boy is so excited that he can't fall asleep.

4. a boy so excited that can't fall asleep

我感觉4是正确的，但是说不出为什么变成只是修饰名词后that从句后就不需要主语了？

请问这是so...that...从句特有的现象吗，还有其他类似的情况吗？