I saw this sentence while doing my homework:

It is still a mystery that how all the birds escaped.

In this sentence, must the ‘that’ be deleted?


In some sentences, you can use just that, and in others just how:

✔ It is still a mystery that all the birds escaped.

This is the equivalent of:

The disappearance of the birds remains a mystery.

You are talking about the simple fact that the birds were suddenly missing.


✔ It is still a mystery how all the birds escaped.

Here, you are questioning their method of escape. This is something more specific than just the simple fact of them being missing.

In some sentences, you can even combine the two words:

✔ Is that how you bake it?

Here, that is being used as a pronoun to stand for something else rather than as a conjunction. In this case, it could also be rephrased in this way:

Is [putting the pie in the oven for an hour] how you bake it?

Since each of the versions of the sentence makes sense with only one of the words, it's even possible that the sentence could be rephrased and expanded to include both versions at the same time:

✔ There are still two mysteries: that the birds escaped and how the birds escaped.

This is a bit strange, and doesn't sound quite normal, but the syntax is still sound.

However, the shortened version of the sentence as it stands is ungrammatical:

✘ It is still a mystery that how all the birds escaped.

It's wrong because that how is acting as a combined phrase in this contrunction, and one that doesn't serve a grammatical purpose. If it were meant to represent multiple things, it would need to be separated by some other conjunction, as in the two-part example sentence involving two mysteries.


You cannot use both that and how together. They are both conjunctions in this sentence, so you should pick one or the other. Normally only one conjunction can be used at a time (with the exception of “and/or”, but I would consider that to be its own conjunction. Besides, the slash makes it clear that the two words are connected).

See definition 1 of how under conjunction

Sense 1a from Merriam-Webster is how the word how would be understood in this sentence. Sense 1b is equivalent to that.

EDIT: addressing some points in the comments. BillJ does not believe how can be a conjunction. If Merriam-Webster's part-of-speech classification is debatable, here's a second opinion: Collins English Dictionary online. Strangely, no "conjunction" definitions appear under (British) English, but five appear under American English. So you can also think of it as an interrogative adverb.

To address Jason Bassford's point, it is fine to use "that how" in certain contexts, like in the question "Is that how you do it?" But in this case that is a pronoun, so there's no problem combining it with how.

  • I wouldn't go along with you on that. "That" is certainly a subordinator (you conjunction), but "how" is an adverb. – BillJ Apr 5 at 17:59
  • Of course you can: Is that how you do it? You have to qualify your statement. While you can use the two together, they can't be used together in this construction. – Jason Bassford Apr 5 at 18:10
  • @JasonBassford in your example, “that” is a pronoun. I can edit my answer if it’s unclear that I mean that “that” used as a conjunction and “how” used as a conjunction are not compatible together. – Mixolydian Apr 5 at 18:18
  • @BillJ “how” can be an adverb but can also be a conjunction. Please see the Merriam-Webster definition linked above. “How are you?” - adverb “I know how you are” - conjunction – Mixolydian Apr 5 at 18:20
  • No, in contemporary grammar, the "how" in the OP's example is an interrogative adverb. Never use dictionaries for grammar! – BillJ Apr 5 at 18:22

First, if you wanted to include "that" in the sentence, you would need another clause.

For example: It is incredible that how all the birds escaped is not better understood.

When you use that as a conjunction, it should be followed by a phrase that could work as a complete sentence on its own. "How the birds escaped" is not a complete sentence, but "how the birds escaped is not better understood" is.

Second, "it is a mystery how" is correct, but "it is a mystery that" is not correct. At least, not anymore. I did find some examples, but they were from a periodical published in 1871.

It is a mystery that the Vaudeville company should leave London at the height of the season, at the very time that the Two Roses was produced last year with so much success. It is a mystery that managers, after so much experience, should still pin their faith on ladies who, whatever their physical qualifications may be, are ignorant of the rudiments of the English language. It is a mystery that they (the managers) should still charge exorbitant prices...

Westminster Papers: A Monthly Journal of Chess


It is still a mystery that how all the birds escaped.

Either the subordinator "that" or the adverb "how" must be dropped, though the resultant meanings are different:

[1] "It is still a mystery [that all the birds escaped]."

[2] "It is still a mystery [how all the birds escaped]."

[1] is an extraposition construction, where the bracketed element is a declarative content clause serving as extraposed subject. The basic non-extraposed version is "That all the birds escaped is still a mystery."

In [2] the bracketed element is a subordinate interrogative clause (embedded question), where the meaning is "The answer to the question 'How did all the birds escape?' is a mystery."

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