QI connects certain verbs and adjectives to question-words, forming indirect questions:
+--QI--+ | | 1. I wonder what book he will read 2. I wonder what will happen 3. I wonder what he will do 4. I wonder when he will come 5. I wonder how big it is +--QI--+ | | 6. I am not certain where he isQuestion-words thus have (QI- or W-). (The W- is used in direct questions, to attach to the wall.) This is conjoined with various connectors. Determiner question-words, like ex. 1 above, use D**w+; subject-type questions (ex.2) use Ss*w+; object-type questions (ex.3) use B*w+; where/when/how questions (ex.4) use Cs+, PF+ or TOn+; adjective questions (ex.5) use EAh+. See entries for these individual link-types.
Verbs ("wonder", "know", "ask") and adjectives ("certain", "clear") that take indirect questions have QI+, disjoined with other complement connectors.
Do not confuse this link with the QN link, which resembles this link in some ways; QN is used for inverted questions, such as
+--QN--+ | | It happened when?Such inverted questions can be formed with almost any verb; whereas indirect questions are fronted by a very limitedset of verbs.
QIi is used by post-processing to enforce the correct use of "filler-it". Certain phrases can only be used with "it" as the subject ("It is not clear what will happen"; *"I am not clear what will happen"); this is enforced in post-processing. QIi is therefore directly analagous to THi (see "SF: filler-it").
QI#d is, similarly, used by post-processing. The issue here is whether or not the QI link should begin a domain. In all indirect questions, it is clear that the indirect question should be in a different group from what precedes. With "where/when/how" constructions ("I wonder when he will come"); the question-word makes a Cs connection to the new subject, beginning an 's' domain; thus the QI link need not start a domain itself. In all other cases, however, the links extending to the right of the question-word (B, S, D, etc.) are not normally domain-starting. In these cases, then, the QI link has to start the domain. Thus we make QI#d+ a domain-starting link, and assign it to question-words like "what" and "who"; "where", "when" and "how" have the non-domain-starting QI+.
Grammar Documentation Page.