What vs. That – Which is Correct?

Both what and that are correct English words, but they serve different grammatical roles. Use what for an unspecified thing or the meaning “the thing that”; use that to identify a specific person, thing, or clause. Choosing between them depends on whether the sentence needs an open idea or reference.

What or That – Which Is Correct?

What and that are both valid American English words, but they are not interchangeable in every sentence. The main difference is specificity.

What often refers to an unknown, general, or unspecified thing. It can also introduce a noun clause meaning “the thing that.” For example, in “I understand what you mean,” what you mean means “the thing that you mean.”

That commonly points to a specific person or thing, introduces a relative clause, or connects a statement to its explanation. For example, in “I found the book that you recommended,” that identifies a particular book and introduces information about it.

The Best Trick to Remember the Difference

Use this substitution test: if you can replace the word with “the thing that,” choose what. If the word points to a particular noun or introduces a clause describing that noun, choose that.

Compare these sentences:

• “I know what she wants.” This means “I know the thing that she wants.”
• “I know that she wants coffee.” This introduces a complete statement: “she wants coffee.”
• “The coffee that she wants is expensive.” This identifies which coffee is expensive.

Word	Part of Speech	Meaning	Example
what	Pronoun, determiner, conjunction	An unspecified thing, idea, or information	Tell me what happened.
that	Pronoun, determiner, conjunction	A specific thing, person, statement, or clause	I think that he is right.

How to Use What

What is used to ask about information, refer to an unspecified thing, introduce an indirect question, or create a noun clause. It can function as a pronoun, determiner, or conjunction. In everyday speech, it is common in both casual and formal English, although the surrounding sentence determines its exact role.

As a question word, what asks for information about an unknown object, idea, action, or identity.

Example 1: What time does the meeting begin?

As a pronoun, what can mean “the thing that” or “the things that.”

Example 2: She explained what caused the delay.

As a determiner, what comes before a noun to ask about its identity, type, or selection.

Example 3: What kind of music do you enjoy?

Etymology of the Word What

What comes from the Old English word hwæt, which meant “what” or “which.” It is related to similar words in other Germanic languages, including German was and Dutch wat. Its basic role as a question word has remained stable for centuries.

How to Use That

That has several important grammatical uses. It can act as a demonstrative pronoun or determiner, pointing to a particular person or thing. It can also introduce a relative clause or connect a reporting verb to a statement.

As a demonstrative determiner, that identifies one specific noun, often one that is farther away in space, time, or conversation.

Example 1: That building was designed by a famous architect.

As a relative pronoun, that introduces information identifying or describing a noun.

Example 2: The article that you sent was very helpful.

As a conjunction, that introduces a content clause after verbs such as think, believe, say, and know. In casual English, it is sometimes omitted.

Example 3: I believe that the proposal will succeed.

Etymology of the Word That

That developed from the Old English demonstrative þæt, which referred to a particular person or thing. It is related to the demonstrative forms this and those. Over time, it also developed conjunction and relative-pronoun functions.

Related Concepts

Understanding clauses, pronouns, and reference makes the difference between what and that easier to recognize.

• Interrogative words: Words such as what, who, where, and when ask questions or introduce indirect questions.
• Relative clauses: A relative clause provides information about a noun. The word that frequently introduces a restrictive relative clause, as in “the car that I bought.”
• Noun clauses: A noun clause functions like a noun within a larger sentence. In “I know what she wants,” the clause what she wants acts as the object of know.
• Demonstratives: Demonstrative words point to specific people or things. That is a demonstrative determiner or pronoun, while what generally asks about or refers to something not yet identified.

Examples from Media and Literature

Writers, journalists, screenwriters, and speakers use both words frequently because they express different relationships between ideas.

• News writing: A report may say, “Officials confirmed that the bridge will reopen,” where that introduces the confirmed statement.
• Instructional writing: A guide may state, “Choose what works best for your situation,” where what means “the thing that.”
• Literary narration: A narrator might write, “She remembered what her father had taught her,” using what for an unspecified body of knowledge.
• Film dialogue: A character may say, “I know that you are worried,” with that introducing a complete clause.
• Descriptive prose: A sentence such as “The path that led to the lake was narrow” uses that to identify which path the writer means.

Practice Exercises

Choose the correct word to complete each sentence.

1. I do not understand ___ you are trying to say.
2. She said ___ she would arrive before noon.
3. The jacket ___ you ordered has arrived.
4. Tell me ___ you need from the store.
5. He selected the option ___ offered the best value.

Answer Key

1. what — It means “the thing that you are trying to say.”
2. that — It introduces the statement “she would arrive before noon.”
3. that — It introduces a relative clause identifying the jacket.
4. what — It asks about an unspecified item or items.
5. that — It identifies which option offered the best value.

What Synonyms

Synonym	Definition
which	Asks someone to identify one or more items from a known or limited group.
whatever	Refers to any thing or choice, without specifying which one.
the thing that	A phrase that often explains the meaning of what in a noun clause.
the information that	A context-specific substitute for what when referring to information.

That Synonyms

Synonym	Definition
which	Can introduce a clause that describes or identifies a preceding noun in some contexts.
the particular	Conveys the specific identifying meaning of demonstrative that.
because	In informal expressions such as “That’s because…,” it introduces a reason, although it is not a universal replacement.
the fact that	A phrase that can introduce a statement or fact in formal writing.

Frequently Asked Questions

Question 1: Is “what” or “that” correct?
Both are correct. Use what when referring to an unspecified thing or meaning “the thing that.” Use that to identify a specific noun or introduce a complete statement or relative clause.

Question 2: What is the difference between “I know what he said” and “I know that he said it”?
“I know what he said” asks or refers to the content of his words. “I know that he said it” confirms the fact that he made the statement, without necessarily giving its content.

Question 3: Can “that” be left out of a sentence?
Sometimes. In “I think that she is correct,” that can usually be omitted: “I think she is correct.” However, omitting it may make a long or complex sentence harder to read.

Question 4: Can “what” introduce a relative clause?
Yes. What can introduce a free relative clause, as in “Take what you need.” This means “Take the thing or things that you need.”

Conclusion

What and that are both correct, but they perform different jobs. Choose what for an unknown idea, question, or “the thing that” construction. Choose that for a specific reference, a relative clause, or a statement introduced after verbs such as think and know. Pro Tip: Using the precise word in professional writing improves clarity and strengthens your credibility.