A&G define the substantive clause as “a clause…used as a noun,” in contrast to the relative clause, which operates in place of adjectives or adverbs.
- I am the man whom you are seeking. (relative clause, as adjective)
- She ascended, as Ariadne ascended with Dionysius. (relative clause, as adverb)
- They warned us this would happen. (substantive clause, as noun)
- She wishes to see you immediately. (substantive clause, as noun)
To tease this out more explicitly, the relative clauses redefine or redescribe ‘man’ and ‘ascended,’ whereas the substantive clauses are effectively an apposition of the verb.
- They warned us this would happen = their warning was ‘this would happen’
- Shes wishes to see you immediately = this is her wish: to see you immediately
A&G refine this, stating that a substantive clause will always apposite a nominative or accusative case. (In the example above, she wishes x and they warned us x would both be in the accusative in Latin.)
English is partial to abstract nouns, where Latin is partial to verbal phrases.
- She demanded an investigation: postulābat ut quaestiō habērētur.
Substantive Clauses Take Four General Forms:
- Subjunctive Clauses
- Indicative Clauses with quod
- Indirect Questions (with the Subjunctive)
- Infinitive Clauses
This fourth form, the infinitive (with possible subjective accusative) is not properly a clause. Still these often replace ut clauses with the subjunctive, and are the mainstay of indirect discourse.
The Essential AG: 560-62