Examine the following sentence:

In my last year in high school, my adviser urged that I apply to several colleges.

This complex sentence is built out of two sentences, one of them (B) embedded in the other (A):

A. In my last year in high school, my adviser urged B.

B. I (should) apply to several colleges.

When these are combined as in the original sentence, sentence B becomes a noun clause introduced by that and becomes the object of the verb urged in sentence A. Now look at the following sentence:

It made a big difference that she wrote a strong letter of recommendation.

Here the two component sentences are C and D:

C. D made a big difference.

D. She wrote a strong letter of recommendation.

In this case, the noun clause formed from sentence D functions as the subject of sentence C so that the combination reads as follows:

That she wrote a strong letter of recommendation made a big difference.

This sentence is gramatically acceptable but not typical. When a long noun clause is the subject, it is usually moved to the end of the sentence. The result is It made a big difference that she wrote a strong letter of recommendation, which inserts the dummy subject It as the subject.