The relative and interrogative pronouns (quī, quae, quod) and (quis? quod?) are originally of the same root, so their older forms overlap in many places. The majority of this post will cover features of Latin you might only live to see once, or never.
The archaic genitive singular of this root is quōius, and the archaic dative singular, quoi.
The form quī is alternative of the ablative in all genders, though most often appears as an adverb (quī, how, in what way, inanway–using the same semantic field as the Greek ὅπῃ), or as quīcum, with whom? However, there are more general instances, even in classical Latin, for instance:
Which chest did the spears pierce: quī pectōre tela / transmittant (Lucan, Bellum Civile 7)
The archaic nominative plural, quēs, is only found in early Latin, though the archaic dative/ablative plural quīs is found in classical poetry.
The relative pronoun is used within a complex sentence to refer to some antecedent in an earlier clause. In Latin, the relative pronoun is decline, and should fit syntactically with its own clause, rather than the case of its antecedent. For instance:
These are not the Droids you’re looking for: haec Droidēs nōn sunt quae quaesis.
The antecedent (nominative) does not align with the relative pronoun (accusative). Also, note how easily Latin shifts and embeds a relative clause:
These are not the Droids you’re looking for: quae quaesis haec Droidēs nōn sunt.
These are not the Droids you’re looking for: haec Droidēs quae quaesis nōn sunt.
These are both acceptable (albeit irregular/poetic) alternatives to the sentence above.
The conjunctive quod sī may be translated ‘but if’ or ‘if however,’ and generally modifies or qualifies a preceding statement with a new condition. It is therefore distinction from the isolate sī, which may produce conditionals without precedent.
If Caesar arrives, we are done for: sī Caesar veneat, pereāmus. (FLV construction)
If Caesar arrives, we are done for, but if we flee, we might survive: sī Caesar veneat, pereāmus, quod sī fugiāmus, fortasse vivāmus.
Caesar is coming. However, if we flee, we might survive: Caesar venit. Quod sī fugiāmus, fortasse vivāmus.
In short, quod sī establishes a conditional in direct relation to some other fact or some other conditional.
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:
Indicative Clauses with quod
Indirect Questions (with the Subjunctive)
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.