A THAT complement refers to some definite event or state. Thus:

(12) I know that John is on duty today

WH- complements involve either (i) whether or if, which enquires about a complement event or state; or (ii) another wh- word (who, what, which, why, etc.), which enquires about some aspect of an event or state. They may be the indirect speech correspondents of questions, e.g. They asked whether he is sick, She enquired who was sick. WH- complements also occur with many verbs not concerned with speaking, then referring to something about which clarification is needed, e.g.

(13) I (don’t) know whether/if John is on duty today

(14) I (don’t) know who is on duty today/when John is on duty

The most typical pattern—with verbs like know, hear, understand, remember, decide and remark—is for a THAT complement to occur in a positive sentence and a WH- one in a negative one. Thus I know that John is on duty today and I don’t know whether John is on duty today. But all WH-complements can be used without a negative, e.g. I know whether John is on duty today. And THAT clauses may be used with a negative, as in:

(15a) I don’t know that John is on duty today

(15b) I didn’t know that John was on duty today

Both (15a) and (15b) would be likely to be used when someone else had made an assertion that John is on duty today. By using (15a), in present tense, the speaker declines to agree with the assertion—(15a) has a meaning not very different from I don’t believe that John is on duty today. Sentence (15b), in past tense, indicates not so much disagreement as surprise—the speaker thought that they knew John’s duty days, and hadn’t realized today was one of them.

Verbs like ponder, speculate, wonder and guess tend to relate to some matter that requires clarification, and typically take a WH- complement without also including not (although they can also accept not), e.g. I wondered whether/when I would get released, She guessed what he would be wearing/who would come.

Doubt has an inherently negative meaning and commonly takes a whether complement in a positive clause; it also takes a THAT complement. There is a subtle difference in meaning—I doubt that he is sick is most likely to imply disagreement with an assertion which has been made that he is sick, whereas I doubt whether he is sick could be used when no one has seriously suggested that he was sick, but the idea has just been floated as one possible explanation for his absence. A negative sentence with doubt may take a THAT complement, e.g. I don’t doubt that he is sick, and is very close in meaning to the positive assertion I believe that he is sick. A negative statement with doubt does not permit a whether complement, *I don’t doubt whether he is sick (it is as if a double negation does not make sense in connection with an appeal for clarification); but note that a negative question with doubt may take a whether complement, e.g. Don’t you doubt whether he ever intended to do it?

Verbs ask, request, and enquire can refer to an act of questioning and are then restricted to a WH- complement clause. Ask—but not enquire—has a second sense of ‘ordering’ and then takes a THAT complement, which will generally include a modal, e.g. I asked that he (should ) clean the stables.

Almost all verbs that take WH- complements also take THAT clauses. The few exceptions include enquire (mentioned in the last paragraph) and discuss, which must refer to some ongoing activity (through an ING complement) or to something about which clarification is sought (through a WH- clause). There are a fair number of verbs that take a THAT but not a WH- complement, e.g. believe, assume, suppose, which make unequivocal assertions, and order, urge, which issue instructions.

The LIKING and ANNOYING types describe an Experiencer’s feelings about a Stimulus, something which is not compatible with a clause that indicates a need for clarification; most verbs from these types take THAT clauses but none accept WH- complements. A number of Secondary verbs take THAT complements but these essentially provide semantic qualification of the verb of the complement clause, and again it would not be appropriate to have a WH- clause in this function. We can thus say both I didn’t know (that) it was you (sc. coming) and I hoped (that) it was you (sc. coming) but only I didn’t know who it was, with the Primary verb know, not *I hoped who it was, since hope is a Secondary verb. (A WH- clause is possible with the Secondary-D verb matter, e.g. Whether he wins matters to me.)

Of the adjectives that take complement clauses, WH- complements are found in subject function with a handful that have negative meanings, e.g. be unsure, be uncertain, and with others when there is a negative in the sentence, e.g. not be certain, as in It is not certain whether he will come. WH-complements may also occur after some HUMAN PROPENSITY adjectives, e.g. He is curious (about) where we’ll sleep tonight, She is interested (in) whether we’ll be allowed in.

(In any investigation of WH- complement clauses one must take special care to distinguish these from fused relatives, such as I like who my daughter married, i.e. I like the person who my daughter married, and I like what you have, i.e. I like that which you have.

We mentioned a group of verbs which commonly take the obligation modal should in a THAT complement; in fact, the meanings of the verbs imply obligation and they may freely omit the should, e.g. He suggests that we (should ) go tomorrow. These verbs include order, urge, recommend and others from the ORDER subtype of SPEACKING; propose, suggest and insist; and a very limited sense of want (as in I want that you (should ) be satisfied ).