A relative clause adds information about a noun or noun phrase. In the engine that stalled, that stalled identifies the engine, and engine is understood as the subject of stalled.
The relative clause is a postmodifier inside the nominal headed by engine. Its subject gap marks the role that the engine is understood to fill.
the test
Put the head noun into the gap
the engine that ___ stalled → the engine stalled
Putting the noun into the open place shows its role inside the relative clause. The pair below holds the head noun and the outer clause still, so the position of the gap is the one visible change.
Which role does the gate fill inside its relative clause?
The gap is the subject of rattled: the gate is the one that rattled.
The gap is the object of damaged: the gate is what the storm damaged.
a different gap relation
An interrogative keeps both ends inside its clause
In He knew what she repaired, what is a fronted phrase inside the interrogative clause and shares an index with the object gap later in that clause.
In a relative clause, the understood role points back to a noun outside the clause. In an interrogative clause, it points to a question phrase at the front of the same clause. The diagrams show similar open positions with different sources.
A relative clause need not begin with a relative word.
The book that I needed and the book I needed contain the same object gap. English often permits the relative word to be absent in an integrated object relative; it is not normally absent when the missing role is the subject.
Ten sentences from this lesson, one decision at a time.