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Frequent learner mistakes involving relative clauses.
Use a defining relative clause when the information identifies the noun and a non-defining clause when the information is additional.
Compare The students who passed the exam received certificates with The students, who passed the exam, received certificates.
Defining relative clauses are normally not separated by commas.
For example, The people who work here are friendly. Adding commas can change the intended meaning.
Non-defining relative clauses are normally separated from the main clause by commas.
For example, My brother, who lives in Canada, is visiting us.
Use who primarily for people and which primarily for things and animals.
For example, The woman who called me and The car which I bought.
Who is common for both subject and object reference in modern English, while whom is mainly used as an object form in formal English.
For example, The person who I met and the formal The person whom I met.
That can refer to people in defining relative clauses, but who is generally the more natural choice for people.
For example, The person who called me is preferred by many speakers, while The person that called me is also possible in defining contexts.
That and which can both introduce many defining relative clauses referring to things, but that cannot normally introduce a non-defining relative clause.
For example, The book that I bought and The book which I bought are standard defining constructions.
Do not normally use that to introduce a non-defining relative clause.
Use who, whom, whose, or which instead. For example, My car, which I bought last year, is reliable.
Determine whether the relative pronoun is the subject or object of the relative clause before deciding whether it can be omitted.
Compare The woman who lives here with The woman who I met yesterday.
A subject relative pronoun normally cannot be omitted.
For example, The woman who lives here cannot become The woman lives here while retaining the same relative-clause structure.
Object relative pronouns can often be omitted in defining clauses, especially in informal and neutral English.
For example, The book that I bought and The book I bought are both standard.
Relative pronouns are not normally omitted in non-defining relative clauses.
For example, My sister, who I visited yesterday, is a doctor. The form My sister, I visited yesterday, is not standard.
Whose expresses possession and should be followed by a noun or noun phrase.
For example, The man whose car was stolen called the police.
Do not use who when the relative clause requires a possessive determiner.
Use The woman whose brother lives here, not The woman who brother lives here.
A preposition directly before a relative pronoun requires an appropriate wh-form rather than that or ordinary who in formal constructions.
For example, the person to whom I spoke and the system on which the software runs.
The preposition in a relative clause must be the one required by the underlying verb, adjective, noun, or expression.
For example, the person I spoke to, not the person I spoke at, unless the meaning specifically requires at.
Do not add a preposition merely because a relative pronoun appears in the clause.
For example, The person whom I met is correct; The person with whom I met is incorrect unless with belongs to the intended meaning.
Use where for place, when for time, and why for reason when the grammatical relationship supports a relative-adverb construction.
For example, The place where I live, the day when we met, and the reason why she left.
Where expresses a place relationship, while which refers directly to the antecedent noun.
Compare The house where I live with The house which I bought.
When expresses a time relationship, while which directly refers to the antecedent noun.
Compare The day when we met with The day which I will never forget.
Why normally refers to a reason and is especially common after the noun reason.
For example, The reason why she left is unclear. The form is redundant in a logical sense but completely standard in English.
Removing an object relative pronoun is different from reducing a relative clause by using a participle or infinitive.
The book I bought uses omission, while the book bought yesterday is a reduced relative clause.
A reduced relative clause must preserve the correct active or passive relationship between the noun and the participle.
For example, the people working here corresponds to people who work here, while the documents signed yesterday corresponds to documents that were signed yesterday.
A participial reduction should clearly modify the intended noun and should not create a misleading or impossible subject relationship.
The reduced participle must logically describe the noun it follows.
What is a fused relative word and normally does not follow an explicit antecedent noun, while that can introduce an ordinary defining relative clause.
Compare I understand what you mean with I understand the explanation that you gave me.
Whoever refers to any person who satisfies a condition, while who normally refers to a specific antecedent noun.
Compare Whoever arrives first can start with The person who arrives first can start.
Whatever normally expresses unrestricted choice, while whichever usually refers to one item from a known or limited set.
Compare Choose whatever you want with Choose whichever of these options you prefer.
The verb in a relative clause must agree with the subject of that clause.
For example, The people who live here are friendly, but The person who lives here is friendly.
Relative clauses should be positioned and structured so that the intended antecedent is clear.
Poor positioning or long chains of relative clauses can make it unclear which noun a relative clause modifies.
Accurate relative-clause usage requires control of clause type, pronoun choice, grammatical role, omission, prepositions, adverbs, reduction, punctuation, and special constructions.
Before forming a relative clause, identify the antecedent, determine the grammatical role of the relative element, and decide whether the information is defining or additional.