In a sentence like scio quid dixeris, is it mandatory to use the subjunctive?
Is it a mistake if I use the indicative instead of subjunctive and say "scio quid dixisti" ?
Were all/most Latinophones following consecutio temporum rules?
In a sentence like scio quid dixeris, is it mandatory to use the subjunctive?
Is it a mistake if I use the indicative instead of subjunctive and say "scio quid dixisti" ?
Were all/most Latinophones following consecutio temporum rules?
The key is, the consecutio temporum doesn't apply to all subordinate clauses—only some specific types that take the subjunctive. You mention Wheelock's description in a comment:
A primary tense of the indicative must be followed by a primary tense of the subjunctive, and an indicative historical tense must be followed by a historical subjunctive tense.
But this doesn't mean that indicative verbs always have to be followed by subjunctive verbs. Rather, it means that when an indicative verb is followed by a subjunctive verb, their tenses have to come from the same sequence. (And even this isn't as absolute a rule as Wheelock says—but that's a separate issue.)
In this particular construction, the indirect question, it's standard to have the subordinate verb be in the subjunctive. But there are equivalent constructions which use the indicative, like a relative clause: sciō quod dixistī. If a construction calls for the subjunctive, then the consecutio temporum comes into play—but it's an effect of using the subjunctive, not the cause of it. (The cause is that this is an indirect question construction, and indirect questions take the subjunctive.)
To answer your second question: yes, in general most Latin authors did follow the consecutio temporum. But, like most rules in linguistics, it wasn't absolute. There are also a number of exceptions written by native speakers, which grammarians have tried to explain away in various ways with varying degrees of success.