One of the most distinguishing characteristics of English relative clauses is that they often 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 main clause or an independent sentence, the major grammatical phrases generally appear in the order:
Subject Verb Object
In contrast, in a relative clause there will be a gap when the targeted grammatical phrase would appear in any position other than the subject position.
Subject Position
First consider relative clauses that target the subject position:
The photographer who lent me a scanner asked me to return it.
In the relative clause, who appears in the subject position before the verb lent, so there is no gap.
Object Position
However, in the next sentence, who represents the object position after visited, so there is a gap in that position (represented for the sake of illustration by a “bullet”).
The photographer who I visited • at the lab lent me a scanner.
Comparing that relative clause with a sentence like I visited the photographer shows that a noun phrase object ordinarily appears in the object position after the verb visited.
Object of Preposition Position
In the next two sentences, the gap appears after the preposition from. Compare I borrowed a scanner from the photographer.
The photographer who I borrowed a scanner from • asked me to return it.
The photographer that I borrowed a scanner from • asked me to return it.
Wh-Phrase with Whose
In the sentence below, the gap occurs after borrowed and represents the entire wh-phrase whose scanner, which has “moved up” to the front of the relative clause. Compare I borrowed her scanner.
The photographer whose scanner I borrowed • asked me to return it.
Entire Wh-Phrase Starts Relative Clause
Similarly, the gap in the next sentence occurs after a scanner and represents the entire prepositional phrase from whom, which has moved up to the front of the clause. Compare I borrowed a scanner from him.
The photographer from whom I borrowed a scanner • asked me to return it.
Gap Can Be Far Away
In more complex sentences that contain a variety of embedded clauses within the relative clause itself, the gap can be very far away from the front of the relative clause. Note the positions of the gaps in the following sentences.
The photographer that my teacher recommended that I borrow a scanner from • refused to lend one to me.
A technique which we were advised to consider using • for our project yielded successful results.
The software my friend told me her brother thought we should buy • through a mail order catalogue was out of stock.