A whole clause can fill a place commonly filled by a noun phrase. In That the ice held surprised us, the clause that the ice held is the subject. A clause used this way is called a nominal clause.
What noun-phrase slot does the clause fill?
That the ice held is the subject: it can be replaced by it.
That the gate failed is a subject complement after was.
the test
Replace first; classify second
What is that doing in each subject?
That selects the noun storm: an ordinary determiner.
That marks a whole clause, and the clause is the subject.
In the first sentence, that selects the noun storm. In the second, it marks a clause containing the storm and arrived. Replacing that whole clause with it supports its subject boundary; its position in the outer frame tells us which noun-phrase-like job it has.
It holds the subject position, while that we left appears in the special extraposed position at the end. The two are related, but it is not a pronoun replacing the clause.In There is a problem, there is the placeholder subject and the noun phrase a problem is the displaced subject. No clause has been extraposed.
Both sentences postpone important information until after the verb, but they do so differently. In the first, it points forward to a clause. In the second, there holds the subject position while the noun phrase a problem follows the verb.
Ten sentences from this lesson, one decision at a time.