Problem Words — That and Which

That and which are frequently used to add secondary clauses — but they are frequently confused!

That
is the restrictive, defining, pronoun
Which
is the non-restrictive, non-defining, pronoun.

Keep these 2 points in mind:

  1. Which phrases don’t change the meaning of the sentence when excluded.
  2. Which phrases introduce an aside, or by-the-way, type of information.

That versus Which

That in the first sentence restricts the statement to cave-dwelling fish: “Among all the fish of the world, some live in caves. Those that live in caves have many adaptations to living in darkness.”

Fish that live in caves display many adaptations to living in darkness.

By using which, the second sentence states: “All fish live in caves and have many adaptations to living in darkness”.

Fish, which live in caves, display many adaptations to living in darkness.

The last sentence illustrates how to use which to introduce information in a “by-the-way” manner.

Astyanax fish, which live in caves, display many adaptations to living in darkness.

That, Which, or an Em dash

When which (or whom) have a preposition, the preposition can come at the beginning of the clause…:

The Healthy Eating Index-2010 (HEI-2010), with which we evaluated diet, is the gold-standard.

…or at the end of the clause:

The Healthy Eating Index-2010 (HEI-2010), which we evaluated diet with, is the gold-standard.

Use that at beginning of the clause…:

The Healthy Eating Index-2010 (HEI-2010) that we evaluated diet with is the gold-standard.

… or highlight the information with an em dash:

We evaluated diet using the gold-standard — The Healthy Eating Index, 2010 (HEI-2010).

Items As Grammatical Subject As Grammatical Object
People who who or whom
People or Objects that that
Objects which which
Table 1: Deciding which pronoun to use is influenced by the antecedent and grammatical role in the sentence it’s position i is related to the who, that, or which depending on the pronoun’s antecedent. given the item referred to.