Wh-questions that target argument positions (various subject and object positions) contain "gaps." A gap is a position within a sentence structure where something appears to be missing in comparison with most other typical sentence structures. In an ordinary English sentence, the major grammatical phrases generally appear in the order:
SUBJECT VERB OBJECT
In contrast, in a wh-question there will be a gap when the targeted grammatical phrase would appear in any position other than the subject position of the main clause. The reason there is a gap is because wh-words and phrases must move to the beginning of a wh-question in English. When the questioned position follows the main verb of the sentence, the moved wh-word or phrase leaves a gap.
Subject Position
First consider wh-questions that target the subject position:
Who developed your company’s inventory control system?
In this sentence, who appears in the subject position before the verb developed, so there is no gap.
Object Position
However, in the next question, who represents the object position after consult, so there is a gap in that position (represented for the sake of illustration by a "bullet").
Who did you consult • regarding the inventory control system?
A possible response to this question might be:
I consulted my business partner regarding the inventory control system.
In this response, which is a statement rather than a question, the noun phrase my business partner appears in the normal object position after the verb consulted.
Object of Preposition Position
In the next question, the gap appears after the preposition with. That is, the gap occupies the object of preposition position.
Who did the company reach an agreement with • ?
A possible response would have a noun phrase in the position of the gap, for example,
The company reached an agreement with the debtor.
A more formal option is to move the entire wh-phrase with whom to the beginning of the question. In this case, the formal whom is used after the preposition. The gap here represents the entire moved prepositional phrase.
With whom did the company reach an agreement •?
Wh-Questions with Whose
In the question below, the gap occurs after lose and represents the entire wh-phrase whose invoice, which has "moved up" to the front of question.
Whose invoice did the bookkeeper lose • ?
Compare a possible answer,
The bookkeeper lost the customer’s invoice.
A wh-phrase with whose is a "possessive" wh-phrase, so the answer to a wh-question containing whose will include a possessive phrase such as the customer’s, the partners’, your, their, etc.
Gap Can Be Far Away
In more complex sentences that contain embedded clauses, the gap can be very far away from the wh-word or phrase. Note the position of the gap in the following question.
What types of inventory did you say that their company uses • in their production process?
The wh-phrase is what types of inventory, which moves to the beginning of the question from its logical position after uses, leaving a gap. That gap is in an embedded clause (that their company uses • in their production process) after the verb say. A response to this question will contain a noun phrase in the object position after uses in the embedded clause, as in the following sentence:
I said that their company uses a variety of metals and plastics in their production process.
In the next question, the gap is within a clause embedded within another embedded clause. This gap represents the subject of the verb phrase should oversee.
Who did you say the company’s lawyer thinks • should oversee accounts receivable?
A possible answer to this question illustrates the levels of embedding in this question:
I said that the company’s lawyer thinks the current payroll manager should oversee accounts receivable.
In order to interpret and respond to wh-questions, a user of English must be able to mentally associate a wh-phrase with its gap.