.“That” is actually five different words spelt the same, of which one is a multi-use conjunction
THE VARIETY OF USES OF “that”
The word that is very much “multi-use” – variable in its grammar as well as its meaning (see 3. Multi-Use Words) and can easily cause confusions as a result (see 68. How Computers Get Grammar Wrong 1). Its main uses may be illustrated as follows:
(a) Caesar took control of Rome. That led to his assassination.
(b) Caesar marched on Rome. That step changed history.
(c) Any changes that appear should be noted.
(d) Learning languages is not that difficult.
(e) Doctors BELIEVE that exercise IS vital.
In (a), that is an ordinary pronoun like it or themselves: it represents either a nearby noun or one understandable from the context of the sentence (see 28. Pronoun Errors). In some sentences it means “the one” (see 63. Constraints on Using “the one[s]”). It can be put into the plural form those. Grammar books call it a “demonstrative” pronoun, reflecting its attention-drawing meaning.
In (b), that is like an adjective: adding to the meaning of a directly-following noun (step). It again has the plural form those. Grammarians give it the technical name “determiner” (see 110. Nouns without “the” or “a”), further sub-classifying it as “demonstrative”.
In (c), that is again a pronoun, but this time “relative”, in other words replaceable by which. It is pronounced differently from the other two uses: with the soft vowel /Ə/ instead of the normal “a” one /æ/. More can be read about it within this blog in the post 34. Relative Pronouns and Commas.
In (d), that is an informal adverb meaning “as much as that”. In Standard English, it usually needs to follow not, hardly, rarely or scarcely.
In (e), finally, that (again with /Ə/) is a conjunction. It resembles the relative pronoun use in its need for a following verb (is above) – a feature that can lead to double meanings (see 182. Structures with a Double Meaning 2, #2). It is distinguishable by its inability to be replaced by which. This is the use I wish to examine more closely in the present post, since it takes a number of different forms and raises some interesting questions.
A useful term for discussing the conjunction uses of that is “that clause”. This means that plus its following verb plus all of the other words that that verb necessitates, such as its subject and adverbials. The underlined words in (e) are a that clause.
“that” CLAUSES THAT ACT LIKE NOUNS
This kind of that clause can occupy the main noun positions in sentences (object, subject, complement, partner of a preposition). It is the kind that is in (e) above. The conjunction nature of that is what overcomes the normal impossibility of a verb with a subject, like is in (e), to be in these positions (see 70. Gerunds). The ability of that to give statements a noun-like quality is especially valuable in list-writing, where different ideas all need the same grammatical form (see 93. Good and Bad Lists).
1. Object “that” Clauses
The that clause in (e) is of this more specific kind, being the object of the verb believe. A characteristic of object that clauses is that that can be left unsaid – though its presence will still be “understood”. When it is present, it cannot normally have a comma before or after it (see 50. Right and Wrong Comma Places).
Not all verbs can have a that clause as their object. A type that commonly does expresses saying or thinking, like believe in (e). The resultant combinations are likely to be either indirect or quoted statements (see 127. When to Use Indirect Speech and 79. Grammar Problems in Quotation Writing). After the imperative forms of imagine and suppose, however, they are example-showing (see 33. Complex Example-Giving). There is no possibility of using that before an indirect question or command (see 57. Indirect Questions in Formal Writing).
Some other verbs also allow object that clauses. One is illustrated by the following sentence from the Guinlist post 65. Verbs that Mean “Must” or “Can”:
(f) Using a spell check will ensure that most errors ARE FOUND.
Additional verbs needing that include MEAN (= “cause”) and LIKE. However, care is needed with the latter (and with related verbs like APPRECIATE, ENJOY, HATE and LOVE) because, rather illogically, it needs it just before that (like it that …). Unsurprisingly, dropping this it is a common error among inexperienced English speakers. It used like this can also have a following when instead of that.
Unfortunately, many English verbs need a different joining device from that to link with another verb after them: either the to (infinitive) form of that verb, or -ing (see 70. Gerunds). For example, if enable replaces ensure above, the next words have to be … most errors to be found; and if facilitate is used, the verb after it has to be being found. WANT and WOULD LIKE are verbs that can cause particular problems: you have to say want/would like (someone/something) to (do something) instead of using that.
The problem that this creates for learners of English, of course, is to know which verbs need which joining devices. There are not many clues. Indirect speech verbs like THINK fairly reliably take that (though exceptions exist – see 79. Grammar Problems in Quotation Writing). The properties of other verbs often just have to be memorised individually.
2. Subject “that” Clauses
That clauses also sometimes act as the subject of a verb, like this:
(g) That air pollution kills is obvious.
However, such uses sound very formal, and are usually rejected in favour of beginning the fact that.
More common is a use at the end of an it sentence, as in this example from the Guinlist post 103. Postponed Subjects in “It” Sentences:
(h) It is not surprising that antibiotics are becoming less effective.
This kind of that clause is a subject placed after its verb (is) and anticipated by a “dummy” pronoun it. A similar kind of sentence can also be made with a starting there (see 161. Special Uses of “There” Sentences). Note, though, that both it and there sentences often have to be made with other words than that.
3. Complement “that” Clauses
Complements are typically nouns, pronouns or adjectives placed after a link verb like BE (see 113. Verbs that Cannot be Passive). The situation where they are most likely to comprise that and a following verb seems to be when the subject of the link verb is a noun of saying or thinking:
(i) THE BELIEF is that success will come eventually.
Other common nouns of this kind include argument, claim, concern, expectation, hope, hypothesis, idea, implication, indication, message, news, plan, point, report, suggestion, theory and word.
Some nouns that are not of saying or thinking can still have a that complement. They include advantage, arrangement, characteristic, clue, curiosity, custom, danger, future, likelihood, outlook, possibility, probability, problem, prospect, reason, risk and situation. Their use with that may be illustrated as follows:
(j) THE PROBLEM is that nobody takes responsibility.
Many other subject nouns can have a complement containing a verb, but only with a joining device other than that – either the to form of the verb (see 119. BE before a “to” Verb) or -ing. Both these and complement that clauses are ways in which BE can give more detail about a general idea (see 117. Saying More Precisely What is Meant).
It is interesting that the non-reporting sentence (j), unlike (i), allows the fact that as well as that. I would suggest that the meanings are slightly different: the fact that implies the reader’s previous familiarity with the fact after it (so that the sentence is just about its equivalence to the subject noun), while that implies no such familiarity. For more about how information familiarity affects language choices, see 156. Mentioning What the Reader Knows Already.
Some of the other nouns like problem allow the same choice. They are underlined above. Exceptions include nouns referring to the future, probably because the future is not a fact.
4. “that” Clauses after Prepositions
It is also possible to use a that clause instead of a noun after some prepositions – lone or within a prepositional verb. However, it must normally go with the fact, e.g.:
(k) Smoking is made attractive BY the fact that it looks “cool”.
(l) The spread of the cold virus mostly DEPENDS ON the fact that people sneeze it out.
An exception to this need to add the fact after a preposition is after in showing how two ideas or points are similar or different (are similar in that …: see 82. Common Errors in Making Comparisons).
Verbs after a preposition can also be in the -ing (gerund) form, but usually the meaning will differ. In (l), for example, the fact that indicates sneezing always happens (it is a fact), whereas -ing suggests that it may sometimes not happen.
“that” CLAUSES THAT ACT LIKE ADVERBS AND ADJECTIVES
That clauses can act in some of the ways that adverbs can. In one, that must be part of a longer conjunction phrase, like this:
(m) IN THE EVENT THAT the rains fail, stored water will maintain irrigation.
The underlined that clause here is adverb-like in the way that most clauses introduced by a conjunction are: they are outside the main noun and adjective positions in the sentence (see 174. Eight Things to Know about Conjunctions, #8). Other that phrases that help to make adverb clauses of this kind include assuming that, given that, granted that, in order that, now that, on condition that, on the understanding that, provided that, seeing that, so that and supposing that.
One other adverb-like use adds to the meaning of a preceding adjective, as in happy that force was unnecessary. Not all adjectives can be used with that in this way, and even those that can may also allow to, -ing or a preposition + -ing (e.g. happy about force being unnecessary), or even no following verb at all (e.g. happy with the situation – see 134. Words with a Variable Preposition).
Many adjectives usable with that are like happy in expressing an emotion or state of mind: angry, convinced, delighted, disappointed, emphatic, hopeful, insistent, jealous, keen, pleased, sad etc.
Others are common after a starting It is… (see 103. Postponed Subjects in “It” Sentences). Most are truth-commenting: (in)accurate, certain, clear, (un)convincing, (in)correct, conclusive, definite, difficult to accept, false, indubitable, obvious, right, sure, (im)possible, (un)likely, (im)probable, rare, true etc.). A few express mental processes: foreseeable, notable, observable, predictable, tenable and understandable.
The final use of that clauses is adjective-like – combining with a noun just in front, like this:
(n) The DENIAL that God exists is no less a belief than its opposite.
Nouns allowing this use tend to be the same as those that combine with a complement that clause, as in (i) and (j) above. One other is no doubt after there is (see 157. Tricky Word Contrasts 5, #1). Nouns that allow that can also normally be used with a preposition and no verb, e.g. the denial of God (without exists).