An easy way to find a noun clause in a sentence is to find the sentence's verb and then ask a question about it. For example, in the sentence "Whether or not to go to the party was weighing on Maria's conscience" we can ask, "What was weighing on Maria's conscience?" It wasn't the party, but "whether or not to go to the party." This is our noun clause. For the sentence "I don't know where we parked the car," we can ask, "What don't I know?" The answer to the question, and our noun clause, is "where we parked the car."
One basic type of noun clause always starts with a question word, such as "what," "where," "when" or "why." Looking for the question word makes these noun clauses easier to pick out. Some examples include, "I have an idea of where the dog might be hiding," where the noun clause is "where the dog might be hiding," or "What he said at the meeting is top secret," where the noun clause is "what he said at the meeting."
Noun clauses that start with the words "whether" or "if" are often used in yes or no questions. You can use the phrase "the answer" to replace these noun clauses and help identify them. For the sentence "Do you know whether Stan took out the trash?" you can replace the noun clause, "whether Stan took out the trash," with "the answer." The sentence becomes "Do you know the answer?"
When a person is thinking or providing an opinion about something, a noun clause starting with the word "that" is often used. For example, in the sentence "I think that I could eat another piece of pie," the noun clause is "that I could eat another piece of pie." Sometimes "that" is omitted, but the verb in the sentence will still indicate mental activity, and you can insert the word "that" to test the sentence. For example, "I believe he has your book" can also be written "I believe that he has your book." "He has your book" is the noun clause in the original sentence. You can also replace "that" noun clauses with the word "so" to test them. "I think that you're wrong" becomes "I think so."