Garden path sentences and the omitted _that_
A sentence can be grammatically correct and still lure the reader into a wrong reading, so that they have to go back and read it again. A reduced relative clause or a word serving as an unmarked part of speech can produce this garden path, and a that dropped from the front of a noun clause impedes comprehension when the clause's verb sits far from its subject.
What a garden path sentence is
To lead someone down the garden path means to mislead them, and a garden path sentence is a grammatically correct sentence that initially lures people into an incorrect interpretation. In Steven Pinker's classic example, The horse raced past the barn fell, the first three words make raced look like the main verb, fell at the end makes that reading impossible, and the reader has to go back to the beginning and reinterpret the whole sentence to reach its meaning: The horse that was raced past the barn fell. Although grammatically correct, a garden path sentence still qualifies as bad writing because it annoys the reader: people do not like being led down the garden path, and they do not like having to backtrack to work out what the writer meant.
Viewed as a whole, a garden path sentence is grammatically correct, but readers do not view sentences as a whole: they read them one word or phrase at a time. So a garden path sentence should be corrected to lead readers in the direction the writer wants them to go. Garden path sentences are easy to find in someone else's work, because they make the reader stop in confusion and backtrack to work out what the writer meant, and harder to find in one's own writing, because the writer knows what they meant.
Why English is prone to garden paths
Garden path sentences are a common problem in written English because English is an analytic language and relies heavily on word order to establish what role each word plays in the sentence. Examples are far more difficult to find in a highly inflected language such as Latin, which uses word endings to indicate parts of speech and case. They are far more common in written than in spoken English, because speakers use prosody, meaning rhythm, intonation or stress, to clarify meaning, and those clues are lost when a sentence is written down. Careful attention to some simple grammatical principles can help a writer avoid them.
Garden path sentences are also common in English because English words can serve as different parts of speech without being marked as such: adjectives can serve as nouns, nouns can become verbs, and verbs can become nouns. For example, The man who hunts ducks out on weekends is revised to The hunter ducks out on weekends, and The old man the boat to The elderly people man the boat. To catch this kind of garden path sentence before it is published, a writer should watch for adjectives used as nouns and verbs that look like nouns.
Reduced relative clauses
A relative clause is a subordinate clause that modifies a noun or noun phrase, and in English it is usually introduced by a relative pronoun: who, whom, whose, whoever, whosoever, whomever, which, what, whatever or that. A reduced relative clause is a relative clause not marked by an overt complementizer such as that or who. In English the relative pronouns are used as complementizers, and dropping them can lead to the garden path effect because the sentence becomes difficult to parse until it has been read to the end.
The garden path effect often results from a reduced relative clause. Many people, in an overzealous attempt to omit needless words, omit relative pronouns, the wh words or that. A reduced relative clause shortens the sentence but may waste readers' time by leading them down the garden path. The cotton clothing is made of is grown in Mississippi is revised to The cotton that clothing is made of is grown in Mississippi, and Fat people eat accumulates to The fat that people eat accumulates.
The omitted that before a noun clause
Ellipsis is the absence of a portion of a syntactic unit in a stylistic effort to be succinct. A that initiating a noun clause is a complementiser, and the noun clause functions as a complement, which completes the meaning of a grammatical unit such as the direct object of a verb. In research writing, the ellipsis of this that is often only a minor distraction, as in Smith hypothesised protein A is insulin, because the meaning is evident when the verb is is adjacent to the subject protein. In some instances, however, over-ellipsis, or ellipsism, impedes comprehension. The distribution of that is extensive in journal articles, both in the Results and in the Introduction and Discussion.
In The new data convinced her students have the maturity to talk about difficult subjects, her students is read as a unit first because it appears to follow a frequent noun-phrase structure, and the reader has to walk that reading back on meeting a verb that shows her and students belong to two sentence elements, a direct object and a complement. The complementiser that, often omitted in speech, where intonation signals the separate elements, can introduce the complement clause and resolve the ambiguity.
The necessity for a that fronting a noun clause appears directly related to the distance between the noun clause's verb and its subject: the greater the distance, the greater the necessity, and ellipsis impedes immediate comprehension when the verb is too distant from the subject.
The it … that pattern is useful to delay an extra-long noun clause from the subject position of a sentence. In It was shown by using fibroblast growth factors in explant cultures the visceral endoderm could be induced to express the liver-specific genes albumin and α-fetoprotein, the that is missing, so the reader's expectation of the pattern is not fulfilled. The revision restores the that after cultures.
One possibility is a large proportion of the vitamin was bound to protein is revised to One possibility is that a large proportion of the vitamin was bound to protein. Its noun clause has a three-word gap between subject and verb, but the explicit logical relation between the subject one possibility and the subject complement a large proportion minimises the effect of omitting that on immediate comprehension. The noun clause, functioning as a subject complement, a renaming of the subject, can be tested by interchanging the subject complement and the subject: That a large proportion of the vitamin was bound to protein is one possibility. The interchange is useful as a test, but a noun clause in the subject position is an infrequent sentence pattern, probably because the overlong subject delays the main verb of the sentence.
In These investigations enabled concluding the experimental conditions necessary to reduce the ejection force were fulfilled, the noun clause is the object of a participial phrase, and the revision restores the that after concluding. Six words stand between the noun clause's subject, conditions, and its verb, were fulfilled, enough to recognise that something interferes with the flow of the sentence pattern. Deleting necessary to reduce the ejection force minimises the gap and reduces the necessity for that: These investigations enabled concluding the experimental conditions were fulfilled.
Nonthematic focus is another consideration: the it … that in a sentence delays the noun rather than placing it in the subject position, a lack of thematisation. For example, It is possible the rate of bone resorption is significantly less for the indomethacin-treated rats can be revised by restoring the complementiser that or by thematisation: The rate of bone resorption may be significantly less for the indomethacin-treated rats. It … that sentences provide sentence variety and emphasise the commentary, such as possibility, but they delay the thematic sentence subject and the main verb of the sentence.
When the two possibilities are compared, reflexively inserting a that regardless of the distance is usually the selected option. Such routine insertion is limited, because an excess of that_s within a sentence is distracting, so _that should be inserted selectively, where it is most necessary.
Redundancy gives another perspective for not using that in such revisions, and over-usage of that may be analogous to over-usage of the: even though grammatically correct, any over-usage, especially in the same sentence, can become distracting. So a hierarchy of specificity can be applied, eliminating the least specific usage. The redundancy is emphasised when the complementiser that occurs along with that as a relative pronoun: in These would be the vitamins that you were taking at the time that you became pregnant not the vitamins that the doctor suggested that you take when you learn that you are pregnant, the first complementiser that seems unnecessary because it is redundant to the second, which seems necessary for emphasis. With the relative pronouns and that first complementiser deleted, the sentence reads These would be the vitamins you were taking at the time you became pregnant not the vitamins the doctor suggested you take when you learn that you are pregnant.
In another type of pattern, such as given that, the that is often ellipsed, probably with just a momentary dissonance, because the reader automatically supplies it. As evidence, given and given that are used interchangeably as a subordinating conjunction meaning accepting that: A video sequence with better quality was transmitted by the VBR channel than by the CBR channel, given (that) the same average rates were used in both channels.