Wh- clauses vs Relative clauses

Question

I am curious about the difference between the sentences I wrote below:

1. what I did was to sit and wait

2. the thing that I did was to sit and wait

Another example with where:

3. this is where I live

4. this is the place where I live

Answer 1

In a comment, John Lawler wrote:

These are called either headless relative clauses or embedded question clauses. They are noun clauses, in that they are clauses with subjects and verbs that act like nouns. Typically they are the subject or object of appropriate predicates: What he did was unforgivable or I really liked what he did. They can refer either to the presupposed answer to the question (What he did next was quite surprising -- these are known as Conjunctive wh-clauses), or to an indefinite answer to the question (What he did next is completely unknown -- these are known as Disjunctive wh-clauses).

Answer 2

1. [What I did] was to sit and wait.

2. The thing that I did was to sit and wait.

3. This is [where I live].

4. This is the place where I live.

Preliminary point: the bracketed constituents in 1. and 3. are not clauses; they are noun phrases with meanings comparable to 'the thing that I did' and ‘the place where I live’, as in your examples 2. and 4. respectively.

The difference you ask about is a matter of the syntax, and 2. and 4. do in fact neatly help to explain the syntax of 1. and 3.

1 and 3. are called fused relative constructions, a special kind of relative where the antecedent and the relativised element are "fused" together instead of being expressed separately, as shown in the 2 and 4. 'What' in 1. is thus simultaneously head of the bracketed NP and object of the relative clause. And in 3. 'where' is both head of the bracketed NP and adjunct in the relative clause.

Your two pairs, then, contrast the syntax of fused relatives and that of the equivalent non-fused relatives, while at the same time demonstrating their identical meanings.

Incidentally, although there is some superficial resemblance between fused relatives and open interrogatives, there are considerable differences, the latter being clauses while fused relatives are noun phrases, as explained above.