That-clause is a dependent (or subordinate) clause introduced by subordinating conjunction that or relative pronoun that. Functioning as all three dependent clauses (only few do that), it is the most ubiquitous clause. Hardly few sentences pass before you encounter one.
Let’s consider that-clause as each of the three dependent clauses.
(Comments that go with examples are in square brackets. Second, that-clauses have been underlined.)
That-clause as relative (or adjective) clause
A relative clause, also known as adjective clause, is a dependent clause that describes nouns like adjective do. The clause, however, always follows the noun it describes.
A relative clause starting with that is used to refer to animals and things. (That is called relative pronoun – and not conjunction or subordinating conjunction – in this use.) It can sometimes be used to refer to people, but prefer who or whom, which are specifically meant for people. Second, the clause is always used in restrictive sense. Note that a restrictive relative clause is an essential part of the sentence as it makes the noun specific, and it comes without commas. Without the restrictive clause, the meaning of the sentence will be lost. Examples of relative that-clause (or relative pronoun that):
The laptop that my brother bought during Black Friday Sale isn’t working. [Comment: The relative clause specifies the noun laptop. Without it, we wouldn’t know which laptop. Hence, it’s a restrictive clause and is not enclosed by commas. The same holds for other examples.]
The heart that loves is always young.
Island countries that are located not too high from mean sea level face an existential threat from rising sea level.
Climate change could trigger war over vital resources that will become scarcer and scarcer.
Vaccines will one day end this pandemic that has taken such an immense toll on us.
Every field – sports, art, or business – has a graveyard of exceptional talent that failed to put in the hard yards.
The family that eats together stays together.
People can do things in love that, to an outsider, may look madness.
Don’t fear the enemy that attacks you, but the fake friend that hugs you.
Uneasy lies the head that wears a crown.
Can that be omitted in relative clauses?
When a relative pronoun is restrictive and is not acting as subject of the clause, it can be left out.
The laptop (that) my brother bought during Black Friday Sale isn’t working. [That is restrictive and it’s not the subject of the clause (my brother is). Hence, you can drop that, shown in brackets.]
The heart that loves is always young. [That is restrictive, but it’s the subject of the clause. Hence, it can’t be dropped. Remember, a relative pronoun has to fulfil both conditions for it to be dropped.]
That-clause as noun clause
A noun clause is a dependent clause that can take the place of a noun in a sentence. A noun clause starting with that can therefore occupy different slots of a noun such as subject, object, and subject complement, with object of verb slot being the most common. (That is called subordinating conjunction in this use.) Here are examples of noun that-clause (or subordinating conjunction that) categorized by seven noun positions in a sentence.
Noun clause essentially complements other elements in a sentence. To understand any of the following seven, especially sixth, in detail refer to the post on complement clause:
That you’ve only 40 percent attendance shows your lack of interest in the course.
That he carried on despite so many difficulties is praiseworthy.
That Mars once had life has long been believed by scientists.
That he chose not to get vaccinated despite comorbidities baffled many.
2. Object of preposition
That-clause can’t be an object of preposition.
3. Object of verb
This is the most common position of that-clause. This is the most common position of that-clause. Speaking or reporting verbs such as admit, advise, agree, answer, argue, complain, inform, say, suggest, and tell are commonly followed by that-clause. So are perception or cognition verbs such as believe, feel, guess, hear, hope, know, see, and think. But most verbs aren’t. Examples:
I confided to my friend that I made an excuse to get leave from the office.
When the last tree is cut down, the last fish eaten, and the last stream poisoned, you will realize that you cannot eat money.
That you’ve only 40 percent attendance shows that you’re not interested in the course. [Both subject and object in this sentence are that-clauses.]
Island countries that are located not too high from mean sea level know that climate change is an existential threat. [The first that-clause is a relative clause describing the noun phrase island countries. The second is functioning as object of the verb know.]
4. Subject complement
The problem is that the test is not full-proof.
Our earlier belief was that the pandemic will go away once enough people are vaccinated.
5. Object complement
Noun clause appearing as object complement is pretty rare. Even rarer is a noun clause starting with that. Here is an example of how it looks.
The manager named the issue that our sales strategy is average.
6. Noun complement or Appositive
That-clause can complement nouns, almost always following the noun it complements. You’ll commonly see them after reporting nouns (comment, remark, statement, claim, argument, response, etc.).
We’ve to face the fact that some of the island countries will disappear because of rising sea level.
There is hope that the pandemic will assume a much milder form in future.
The politician’s claim that the elections were not fair proved false.
His comment that the players are spoilt because of excessive money was uncalled for.
7. Adjective complement
That-clause can complement adjectives, almost always following the adjective it complements. It is used after ‘linking verb + adjective’ to express opinion or feeling.
I’m ready to take a plunge, but it’s obvious that you’re undecided. [It’s obvious is incomplete in meaning without the underlined adjective-complement clause. Also note that the clause follows the adjective obvious, and the adjective itself follows the linking verb is.]
It is clear that the third wave of the pandemic will arrive sooner or later.
James has performed well in the state championship, but I’m not sure that he’ll achieve the same result at the national level.
I’m happy that you’ve made full recovery from your illness.
Can that be omitted in noun clauses?
That can’t be dropped in that-noun-clauses except where the clause is object of verb or adjective complement. Let’s take both the exceptions.
Can that be omitted in that-clause following verbs?
The answer to this question is bit nuanced.
That can be dropped if the subject of the main clause and that-clause is the same person, which is quite often the case in reported speech:
She said (that) she won’t attend school tomorrow. [Same person: she and she. Brackets around that implies that it can be dropped.]
Mac argued (that) he wasn’t guilty. [Same person: Mac and he.]
Otherwise, that can be dropped as long as it doesn’t sound odd. Note that we’re talking of writing. In speaking, any oddity arising out of dropped that is much less likely to be noticed. Few sentences from the above examples with that dropped:
Scientists believe (that) Covid is going to stay with us for at least few years.
Studies show (that) human productivity drops off significantly after 5-6 hours of intense mental work.
The lawyer argued that his client was not found in possession of the contraband.
The third sentence sounds bit odd without that, and it would be better to retain that. It’s better to be on the conservative side than to cause a momentary reading-stumble for your readers.
However, that is usually not dropped:
1. If the sentence contains more than one parallel that clause
It’s better to retain both that’s so that readers can easily see the beginning of each that-clause. Example:
The main point in the above three examples is that both the things being compared are clearly mentioned and that each of them satisfies the definition of a metaphor.
2. If dropping that leads to confusion
In this sentence dropping that may lead some to think that his failure in the exam is object of verb admitted.
The student admitted that his failure in the exam was a result of his bad company.
This sentence though won’t create similar confusion, and hence that can be dropped (it doesn’t sound odd).
The student admitted (that) he plagiarized for his paper.
3. If the verb is passive
The organizers were told that no one will be allowed without an invitation.
The police were informed that a suspicious person was seen outside the museum.
4. If that-clause starts with an introductory phrase
Avoid dropping that in this sentence because of the presence of introductory phrase despite all human efforts.
Scientists believe that despite all human efforts, Covid is going to stay with us for at least few years.
Without the introductory phrase though, you can drop that.
Can that be omitted in that-clause following adjectives?
That is usually dropped in that-clauses following adjectives. If you drop that in the above examples, you’ll be left mostly with fine sentences. Examples:
It is clear (that) the third wave of the pandemic will arrive sooner or later.
James has performed well in the state championship, but I’m not sure (that) he’ll achieve the same result at the national level.
That-clause as adverb clause
An adverb clause is a dependent clause that expresses adverbial meanings such as time, place, degree, and reason. That can be used after so and such to express adverbial meaning of result or consequence of something. It can also be used as so that to express purpose. (That is called subordinating conjunction in this use.) Examples of adverb that-clause (or subordinating conjunction so that and such that):
He was so nervous before the interview that he forgot to take his resume in. [Result]
The test was so tough that most students were scratching fiddling with their pens even after fifteen minutes. [Result]
It’s better to accomplish significant part of day’s work in the morning so that you’re on top of your schedule and not chasing things later in the day. [Result]
In many countries, the wheels of justice move so slow that sometimes it takes decades to pronounce a judgment. [Result]
Plan your schedule carefully so that you waste as little time as possible. [Result]
Some are so obsessed with creating the best possible product that they take too long to finish, by when its utility goes down. [Result]
And now we’ve gone so much deep into this niche product that it’s taking forever to finish. [Result]
We work so that we can be meaningfully engaged and earn a living. [Purpose]
It was such bad news that no one in the room spoke about it. [Result]
The impact of the pandemic has been such that very few industries haven’t faced difficult times. [Result]
Can that be omitted in adverb clauses?
No. Subordinating conjunctions in adverb clauses, including that, can’t be dropped.