Noun clause vs. Relative clause

Noun clause and relative clause differ in few ways:

1. Function in sentences

1.1. Function of noun clause

Noun clause is a dependent clause that functions as noun in a sentence. Since it functions as noun, it can occupy nominal positions such as subject, direct object, object of preposition, and subject complement. (It rarely functions as indirect object or object complement.) Examples:

Being a subject, object, or complement, a noun clause is an essential part of a sentence: Dropping it will result in a non-grammatical sentence. The sentences we just saw will look like these if we drop the noun clause:

1.2. Function of relative clause

Relative clause, or adjective clause, is a dependent clause that functions as an adjective in a sentence. It modifies noun phrase and usually follows it. Examples:

Being a modifier (adjective), relative clause is not an essential part of a sentence: Dropping it will result in a grammatical, though less informative, sentence:

2. Marker words

If you noticed in the earlier examples, the two dependent clauses have been introduced by words such as how, when, why, and that. They’re called marker or trigger words. They mark the beginning (or they introduce) the two dependent clauses.

Marker words work differently in the two clauses.

2.1. The two clauses have different set of marker words, many of which overlap.

Noun clauses are introduced by marker words that, if, whether, who, whom, whose, what, which, when, where, why, and how. Out of these, that can sometimes be dropped when the clause functions as direct object of verb. Example:

Experts believe (that) consistent sleep patterns lead to better overall health. [You can retain or drop that.]

Relative clauses are introduced by marker words called relative pronouns (who, whom, that, and which), relative determiners (whose), and relative adverbs (when, where, and why). (Contrary to what many resources on the internet mention, whose is not a relative pronoun.) Out of these, all but whose can be dropped under certain conditions. (Read: When relative pronoun can be dropped and when relative adverb can be dropped?) Example:

The early 2000s was the era (when) technology began to transform daily life significantly. [You can retain or drop when.]

If you notice, all marker words of relative clause can introduce noun clause as well. Here are examples of each overlapping marker word so that you can understand how they introduce two different clauses. Where the clause is noun, you can drop it and still have a meaningful sentence. Where the clause is noun, you can replace it by a noun phrase. Where the clause is relative, it modifies a noun phrase and usually follows it. Dropping it would result in less information about the noun phrase.

Who

She didn’t tell me who made the final decision. [Noun clause as direct object. Dropping it will result in an ungrammatical sentence. Alternatively, you would be able to replace it by a noun phrase and get a meaningful sentence: She didn’t tell me the news.]

The person who called earlier left a message for you. [Relative clause. The relative clause modifies and follows the noun phrase The person. If we drop it, we would lose some information about the person.]

Whom

We don’t know whom they will choose for the position. [Noun clause as direct object]

The person whom you met yesterday is my cousin. [Relative clause]

Whose

Whose performance is the best will be decided soon. [Noun clause as subject]

The author whose book we read will be at the event tonight. [Relative clause]

That

He didn’t realize that the meeting had been rescheduled. [Noun clause as direct object]

I finally watched the movie that everyone has been talking about. [Relative clause]

Which

The judge will determine which team wins the competition. [Noun clause as direct object]

The painting which was displayed in the gallery attracted many visitors. [Relative clause]

When

Please tell me when you plan to leave. [Noun clause as direct object]

I’ll never forget the moment when we first met. [Relative clause]

Where

The decision depends on where the event will take place. [Noun clause as object of preposition. You can, for example, replace the noun clause by a noun phrase: The decision depends on location.]

The café where we met has since closed down. [Relative clause]

Why

She didn’t explain why she was late. [Noun clause as direct object]

The reason why she left early is still unclear. [Relative clause]

2.2. Marker words are different part of speech in the two clauses.

In a noun clause, marker words who, whom, whose, what, which, when, where, why, and how can be pronoun, determiner, or adverb. (Specifically, they’re called interrogative pronoun, interrogative determiner, and interrogative adverb.) Marker words that, if, and whether though are treated as subordinating conjunction. Examples:

In a relative clause, marker words who, whom, that, and which are pronoun; marker word whose is determiner; and marker words when, where, and why are adverb. (Specifically, they’re called relative pronoun, relative determiner, and relative adverb.) Examples:

In this post, we looked at the difference between noun clause and relative clause. For difference between other dependent clauses, refer to:

• Relative clause vs. adverb clause
• Adverb clause vs. noun clause