A THAT complement essentially refers to some activity or state as a single unit, without any reference to its inherent constitution or time duration. In contrast, an ING complement refers to an activity or state as extended in time, perhaps noting the way in which it unfolds. Compare:

(16a) I heard that John had slapped his sister

(16b) I heard John(’s) slapping his sister

(17a) He thought that Mary would apply for the job

(17b) He thought of Mary(’s) applying for the job

Sentence (16a) states that I heard a piece of news, whereas (16b) implies that I overheard what actually happened—the noise of palm hitting arm, the girl’s screams, etc. And (17a) states an opinion that the application would be made, where (17b) may be taken to relate to how she would go about it—searching for pencil and paper, wondering how to frame the letter, then buying a stamp.

Some verbs are, by virtue of their meaning, restricted to only one of these complement constructions. Suppose, which relates to whether or not something might happen or have happened, may only take THAT. The meaning of describe implies reference to the unfolding of an activity, and this verb is restricted to an ING complement.

For many verbs THAT and ING complements show considerable overlap in meaning and use. But there is always an implicit—or potential—semantic contrast, along the lines we have described. Consider propose, in Each of (18a–c) could have a THAT or an ING complement.

(18a) I propose (our) walking from John o’ Groats to Land’s End to raise money for charity

(18b) I propose that we (should) do the walk in the spring

(18c) I propose that we (should) forget the whole thing

Each of (18a–c) could have a THAT or an ING complement. The ING alternative sounds fine in (18a), since it introduces the idea of a continuous activity. For (18b) a THAT construction is preferred—the walk is now being referred to as an ‘event’ and a time suggested for it. Sentence (18c) again refers to the walk as a ‘unit’; the ING alternative to (18c), *I propose (our) forgetting the whole idea, sounds particularly infelicitous.

There is another factor which motivates the choice between THAT and ING complement clauses: only an ING construction allows the complement clause subject to be omitted when it is coreferential with an appropriate main clause constituent. There is, in most styles of English, a preference to omit a repeated constituent and—all else being equal, or nearly equal—an ING complement may be preferred to a THAT one for this reason. Thus, if I were the only person planning to walk the length of Britain, then I propose doing the walk in the spring might be preferred to I propose that I (should ) do the walk in the spring. Or, consider regret, which can take a THAT or ING complement, as in:

(19a) I regret that they didn’t walk out of that film when the violence started

(19b) I regret their not walking out of that film when the violence started

Native speakers find (19a) and (19b) about equally acceptable. However, when the complement clause subject is coreferential with main clause subject they exhibit a preference for the ING construction, I regret not walking out ..., over the THAT one, I regret that I didn’t walk out ... (although the latter is still considered grammatical).

The predicate of a THAT complement clause must involve a tense inflection, and can include any of: a Modal, previous aspect have, imperfective aspect be. An ING clause may not include tense or a Modal. The time reference of an ING clause can often be inferred from the lexical meaning of the main verb, e.g. in (18a) it is taken to refer to something projected for the future because propose has an inherently future meaning, and in (19b) to something which took place in the past, because of the meaning of regret. Note that past time can be shown in an ING or TO complement clause by the have auxiliary. An alternative to (19b) is I regret their not having walked out . . . But to include have here would be a little pedantic, since the past tense reference of the ING clause in (19b) can adequately be inferred from the meaning of the main verb regret.