Dropping or retaining the complementizer that
We can drop or retain the complementizer (or subordinator) that without affecting the meaning of the clause. Certain factors appear to favor one choice or the other.
Omission of that is favored by the following factors:
(a) when think or say is the main verb – I think it’s worth doing, Tim says it’s easy
(b) when the subject refers to the same entity in the main clause and in the that-clause, as in Tim promised he’d do it
(c) when there is a pronoun rather than a noun head in the that-clause (I think I’ll have a cola, She knew he would do it).
It has also been suggested that I think and I know, for example, are not main clauses at all, but are better analyzed as epistemic, evidential or evaluative parentheticals, while what is traditionally classed as the complement clause in fact carries the main proposition. This view is based on two pieces of evidence: the verb + its subject can be placed parenthetically after the clause – I’ll have a cola, I think; He’ll do it, I know – and the tag-question relates to the complement clause, not to the main clause – I think she’ll have a cola, won’t she? (not *don’t I?).
Retaining that after a verb is favored by:
(d) coordinated that-clauses: Many people believe that big is best and that war is right.
(e) passive voice in the main clause: It is believed that peace is in sight.
(f) a NG or PP (or clause containing a NG) placed between the main clause and the that-clause: Can you prove to the commission that the effects are not harmful?
Overall, that is omitted most in informal spoken registers, which is where the ‘abc’ factors tend to cluster, while the subordinator is retained most in formal written registers, which are characterized by the ‘def’ factors. These are not strict divisions, however, as even formal registers nowadays are often a mix of the formal and the less formal. The following short extract from The Peacemakers illustrates the tendencies:
People have often assumed that, because Lloyd George opposed the Boer War, he was not an imperialist. On the contrary, he had always taken great pride in the empire but he had never thought it was being run properly.
That-clauses do not follow prepositions in English and consequently cannot realize the PC function. Instead, one of three solutions is adopted: a) the preposition (e.g. on) is omitted; b) the preposition is retained and is followed by anticipatory it, or c) the fact can be inserted before a that-clause with a factual meaning:
(a) He insists that we all go.
(b) He insists on it that we all go.
(c) You must allow for the fact that they are handicapped.
