Noun clauses are dependent clauses that function as nouns in the independent clause.

• They can be placed as subjects or as objects to the independent clauses. 
  • The world knows what caused global warming. > a noun clause as an object to the independent clause
  • What caused global warming is widely known. > a noun clause as a subject to the independent clause
• Noun clauses follow certain adjectives and nouns as the following:
  • Environmentalists were delighted that measures are to be taken to eliminate the dangers of global warming.
  • Every citizen of the world was of the view that governments should do more to tackle the issue of global warming.
• There are three types of noun clauses.
  1. that clauses
  2. if/whether
  3. question clauses, which begin with a question word such as who, what, where, when, or how
• Never use a comma after either the independent or the dependent clauses.
• When the independent clause is a question, we put a question mark at the end. If it is a statement, we put a period at the end.

1. That Clauses

It is a dependent noun clause that begins with that.

• • There is little hope that global warming will be reversed.
• We can remove that without affecting the meaning of the sentence.
  • There is little hope global warming will be reversed.
• Also, we can start with that clause, but we can never omit that in that case.
  • That global warming will be reversed is debatable.
  • Global warming will be reversed is debatable. 

That clauses can happen in different places in a sentence.

1. After the independent clause verb, which is the most common location. In this case, it functions as an object.
   • The Paris Agreement states that world leaders should take measures to limit global warming. 
2. After certain adjectives such as happy, delightful, glad, proud, pleased, sad, worried, surprised, sorry, certain, and sure
   • Scientists were surprised that polar ice melts at a faster pace than expected.
3. After certain nouns, that clauses are common. These nouns are idea, theory, view, thought, claim, notion, impression, belief, assumption, claim, assertion, statement, fact, and opinion.
   • The fact that fossil fuel is still widely used is disappointing. 
4. That at the beginning of a sentence can be grammatically correct; however, it is a little awkward and unnatural to a native speaker’s ear. Native speaker tend to start their sentences with it in this case.

• • That global warming’s ramifications will be mitigated is debatable. > unnatural
  • It is debatable that global warming’s ramifications will be mitigated.  > natural.
• Also, the verb that follows it (except for be or any intransitive verb seem or appear) is often turned into the passive form when writing academically.

2. That Clauses in Reported Speech

Noun clauses are widely used in reported speech. This can be called reported speech, indirect speech, or indirect quotation.

• If the main clause verb is present simple, present perfect, or future, then the verb in the noun clauses is in the tense that expresses the meaning of the main clause. 
  • World governments agree that global warming is a pressing matter.
  • They have decided on reversing measures hoping that those proposed measures will mitigate the impact of global warming on our planet. 
• On the other hand, if the main clause verb is in the past tense, the verb in the noun clause is usually in the past tense as well. 
  • Last week, the world leaders gathered to discuss the issue of global warming. And, they decided to impose new laws that would help reduce Carbon dioxide emissions into the atmosphere. 
• With the exception that when talking about facts, the verb stays in the present tense in the that clause. 
  • Scientists proved that global warming is now irreversible. 

3. Subjunctive Noun Clauses

The subjunctive form is the base form of a verb, for example, be, go, know. With some verbs and adjectives, we have to use the subjunctive form.

• Verbs: advise, insist, request, ask, order, require, command, suggest, prefer, demand, urge, propose, direct, and recommend
• Adjectives: advisable, mandatory, desirable, necessary, essential, urgent, important, vital
  • It is advisable that every individual reduce their energy consumption. 
  • Power providers urge that people not leave the lights unnecessarily. 
• The subjunctive happens when the independent clause verb is in the passive voice.
  • It is recommended that dwellers of big cities use public transportation. 

4. If/whether Clauses

It is the dependent noun clause that starts with the subordinator whether or if. If is less formal than whether. The phrase or not is optional. It could be added with whether before or after the independent clause. But, it can only be used at the end of the if clause.

• • Scientists argue whether or not global warming is reversible. 
  • Scientists argue whether global warming is reversible or not.
  • Scientists argue whether global warming is reversible. 
  • Scientists argue if global warming is reversible. 
  • Scientists argue if global warming is reversible or not.
• If/whether clauses are statements not questions, and are formed as follows:
  • Does global warming have severe consequences?
  • I would like to know if global warming has severe consequences. 
• The same reporting speech rules apply here.

5. Question Clauses

These are dependent noun clauses that begin with a subordinator such as who, what, when, where, why, how, how much, how long, and so on. The subordinator could either be the subject of the dependent clause or not. 

• • Scientists know what caused global warming > What is the subject of the dependent clause. 
  • Scientists do not know when global warming started. > Global warming is the subject of the dependent clause.
• The word order is a statement order (subject + verb) not a question order (verb +subject). No helping verbs are used.
• The same reporting speech rules apply here, too.