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"What" vs. "That"

One common mistake among many English learners is using "what" and "that" interchangeably in the middle of the sentence. Many English learners, especially learners whose first language is Spanish, struggle to learn the difference. The word "that" should be used to introduce clauses, whereas "what" should be used when it is the object of the clause you will introduce.

Let us take the following examples about someone choosing between two classes in college:

  • Mary liked that the professor of the chemistry course was very nice.
  • Mary liked what her friends said about the biology course.

While the words "that" and "what" may seem to fill an identical role, they are very different. In the first example, "that" is not the object of the clause it introduces. It's role is only to introduce clauses. On the other hand, in the second sentence, "what" is the object of what Mary's friends said. Mary's friends said something about the biology course. The sentence doesn't tell us what was said, only that Mary liked it. Note the following example:

  • Mary liked that her friends' talk about the biology course.

In this case, the object of what her friends said is "good things," not "that". Also note that Mary doesn't like the "good things," Mary likes that her friends said good things. In this sentence, she values her friends opinions, not only the information itself. If we wanted to say that Mary liked the "good things," we would say:

  • Mary liked the good things her friends said about the biology course.

Now Mary likes "the good things" her friends said. Note that "the good things" is the object of both what Mary liked and what her friends said. Lastly, if we wanted to replace the "the good things" with a pronoun, we would use.... "what."

  • Mary liked what her friends said about the biology course.

And that is the key difference between "that" and "what."

Another version of explanation

You should know: in this case “what” is a pronoun, “that” is a conjunction.


  • I know what they won.
The pronoun “what” is the object of the clause “what they won”; the clause “what they won” is the object of the sentence “I know what they won”. “What” replaces the noun, phrase or clause like “the price”. “Won” is a transitive verb here. Other pronouns with the same function are “who” (with transitive verb), “which” (with transitive verb), “when” (with intransitive verb), “where” (with intransitive verb), “why” (with intransitive verb), “how” (with intransitive verb).
  • I know (it) that they won.
The conjunction “that” makes the clause “they won” equal to the omitted cataphor “it” in the clause “I know it”. “That” indicates the phrase or clause (something can't be explained only by a noun). “Won” is an intransitive verb here. “That” can also be omitted sometimes.

If you understand these sentences, you have understood the differences of “what” and “that”:

  • I know that they won, but I don't know what they won. What did they win? The ECWC?
  • I know what they won. They won the ECWC. Of course I know that they won. I even know what they won.

If you still can't understand, copy these sentences and paste into Google Translate.


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