A relative clause is a sub-clause, which modifies phrases, pronouns, or nouns. At the minimum, the clause should consist of a verb and a subject. These types of sub-clause are mostly introduced by a relative pronoun such as who, whom, that, which, whose, where, or when. Relative pronouns establish the connection to the phrases, pronouns, or nouns being modified, and therefore, appear at the beginning of the defining relative clause. However, a relative pronoun is sometimes optional in which case; it can be omitted a verb.

The omission of a relative pronoun in a sentence depends on whether it is the subject or not. Typically, “who “, ‘which’ or ‘that’ can be dropped when it is not the subject. In some cases, the relative pronoun appears just before the verb of the relative clause. For example, in the sentence “The topic which interested me was linguistics,” the relative pronoun “which” precedes the verb “interested,” as the subject in the sentence. Therefore, it cannot be omitted. Another example of a sentence in which a relative pronoun serves the purpose of a subject is “A doctor who is rude to you will not give you good medical treatment.” In this situation, “who” is the subject and “is” is the verb. Thus, “who” cannot be left out. The explanation holds for “whom” in “Sheila interviewed the author about whom she was writing an article.”

However, relative clauses can allow the zero option if the relative pronoun is not the subject in the sentence. In “The woman who I saw was Alice,” “who” is not the subject. Moreover, unlike in the previous examples, there is no verb after “who.”  Therefore, without this relative pronoun, the resulting sentence; “The woman I saw was Alice” still makes complete sense. Similarly, removing “which” from “Italy and Uruguay are the countries which Cynthia wants to visit” yields “Italy and Uruguay are the countries Cynthia wants to visit,” which is likewise grammatical. The same explanation applies to “Amanda’s phone, which she had just bought, disappeared into the lake,” from which case “un-marking” the relative clause does not affect the grammatical completeness of the sentence. Also, “who” can be omitted in “I’ve never met a linguist who I didn’t like.”

Nevertheless, some sentences may demand that if the relative pronoun, which does not serve as a subject, is omitted, the comma that precedes it has to be removed as well. For example, in the sentence “Mark Ruffalo was in the film that won Best Picture, which I forget the name of,” “which” can be dropped. The resulting sentence is “Mark Ruffalo was in the film that won Best Picture, I forget the name of. The comma before the unmarked independent relative clause does not alter the meaning but makes the sentence ungrammatical.

In conclusion, clauses that can use the zero option tend to have some properties in common. The most important feature of such relative clauses is that they contain a relative pronoun which functions as the subject in the sentence. For this reason, when the relative pronoun is omitted, a verb becomes the first word in the clause making the sentence ungrammatical. By contrast, in a relative clause, which allows the zero option, the relative pronoun is not the subject. Therefore, removing the relative pronoun has no impact on the grammar of the sentence.