Below are explanations for the two Nominal Clause Samples:

Nominal Clauses from The Heart of Darkness

Forthwith what change came over the waters was the serenity that became less brilliant but more profound. The old river in its broad reach rested unruffled at how the day declined, after ages of good service done to the race that peopled its banks, spread out in the tranquil dignity of a waterway leading to the uttermost ends of the earth. We wondered why the venerable stream did not suffer the vivid flush of a short day that comes and departs for ever. And indeed nothing is easier for a man who has, as the phrase goes, 'followed the sea' with reverence and affection, than to evoke what lingered upon the lower reaches of the Thames.

As the interactive exercise instructs, the first step is to highlight the Main Verb Phrases to determine the clauses for analysis:

Forthwith what change came over the waters was the serenity that became less brilliant but more profound. The old river in its broad reach rested unruffled at how the day declined, after ages of good service done to the race that peopled its banks, spread out in the tranquil dignity of a waterway leading to the uttermost ends of the earth. We wondered why the venerable stream did not suffer the vivid flush of a short day that comes and departs for ever. And indeed nothing is easier for a man who has, as the phrase goes, 'followed the sea' with reverence and affection, than to evoke what lingered upon the lower reaches of the Thames.

There appears to be four nominal clauses in the passage:

1. WHAT CHANGE CAME OVER THE WATERS WAS THE SERENITY: Nominal Clause - WHAT is the dependent marker and precedes the subject - Noun Slot = subject
2. HOW THE DAY DECLINED: Nominal Clause - HOW is the dependent marker and precedes the subject - Noun Slot = object of the preposition (AT)
3. WHY THE VENERABLE STREAM DID NOT SUFFER: Nominal Clause - WHY is the dependent marker and precedes the subject - Noun Slot = direct object (WONDERED)
4. WHAT LINGERED: Nominal Clause - WHAT is the dependent marker and replaces the subject - Noun Slot = direct object (TO EVOKE)

Challenge Sample

What was left of Margot's mind reeled. She could hear the listless, scraping sounds of fingertips against the door. It haunted her to consider that a sea of faces lurked outside. They were faces she once knew in a different light, but now they were putrid reflections of squalor and decay. There was only what she could glean from their lifeless eyes: the grim reminder that she was alone. She tried to ignore what appeared before her, but she truly believed she was the last bastion of humanity. Death awaited her outside.

Again, the first step is to highlight the Main Verb Phrases to determine the clauses for analysis:

What was left of Margot's mind reeled. She could hear the listless, scraping sounds of fingertips against the door. It haunted her to consider that a sea of faces lurked outside. They were faces she once knew in a different light, but now they were putrid reflections of squalor and decay. There was only what she could glean from their lifeless eyes: the grim reminder that she was alone. She tried to ignore what appeared before her, but she truly believed she was the last bastion of humanity. Death awaited her outside.

For this sample, there appears to be five nominal clauses in the passage:

1. WHAT WAS LEFT OF MARGOT'S MIND: Nominal Clause - WHAT is the dependent marker and replaces the subject - Noun Slot = subject
2. THAT A SEA OF FACES LURKED OUTSIDE: Nominal Clause - THAT is the dependent marker and precedes the subject - Noun Slot = direct object (TO CONSIDER)
3. WHAT SHE COULD GLEAN FROM THEIR LIFELESS EYES: Nominal Clause - WHAT is the dependent marker and precedes the subject - Noun Slot = subject complement
4. WHAT APPEARED BEFORE HER: Nominal Clause - WHAT is the dependent marker and replaces the subject - Noun Slot = direct object (TO IGNORE)
5. [THAT] SHE WAS THE LAST BASTION OF HUMANITY: Nominal Clause - Deleted THAT is the dependent marker and precedes the subject - Noun Slot = direct object (BELIEVED)

Tips to Keep in Mind

The KEY MARKER for identifying Nominal Clauses is the interrogative or nominal-THAT that replaces or precedes the subject of the clause. Like a relative THAT, a nominal-THAT when preceding the subject can be deleted. Again, even though the nominal is deleted, the clause is still nominal and still functioning nominally. So whenever you see a clause, especially, after a Main Verb Phrase (or a non-finite verb) without a dependent marker or punctuation (such as SHE TRULY BELIEVED SHE WAS THE LAST BASTION OF HUMANITY above), you should suspect a deleted THAT.