quae sunt isdem in erratis fere quibus ea, quae de Platone dicimus
To be honest, I'm not seeing where the breakdown occurs, but perhaps you're not seeing the sequence of three relative clauses? Barry is a teacher, and perhaps he can be of more help, but let me try.
A relative clause is a subordinate, dependent clause that must have a subject and a verb, correct? As you read any sentence, whenever you encounter a relative pronoun, you know you've got a relative clause that will require its own subject and verb. Let's start with that understanding.
quae relative pronoun, neuter nominative plural, the subject of sunt; the antecedent is the collection of phrases that make up Xenophon's summary of Socrates's doctrines.
sunt is the verb of this clause, it works like a linking verb in English.
Correct, though I don't know why you feel you need to say it's like a linking verb in English. It just happens to be the verb of this relative clause, period.
ea seems to be predicate nominative, it agrees with quae in gender and number.
So as I read it, these three words produce a clear meaning, "Which are . . . the ones".
But then I get to isdem in erratis fere quibus. I can't work out the relations of these words. isdem in erratis fere must mean something like "in the same [state of] error", but I don't see how to fit in quibus.
This seems to be where the breakdown occurs. I don't think you see that quibus
is a relative pronoun and, being a relative pronoun, it is introducing a (second) relative clause, which must have its own subject and verb. Let's proceed Dexter Hoyos-like. quae sunt
: You have a complete subordinate clause, which in Hoyos' terminology is one type of word-group. isdem in erratis
: You have a prepositional phrase, another type of word-group, modifying quae sunt
: Modifies isdem in erratis
. You can think of fere
as a separate phrase and word-group, or simply treat isdem in erratis fere
as the word-group.
quae sunt isdem in erratis fere
: Stop there. You have a completed relative clause. The sentence could end there, and it would make sense, if isdem
had a clear antecedent ("which things [recounted by Xenophon] are almost in the same errors [i.e., types of errors we were just talking about]". But in this sentence that doesn't feel right. It just feels like isdem
is going to have a subsequent explanation. And indeed as your eye moves ahead, voilà, there is no period ending the sentence. Instead there is quibus
. Then there is quibus ea
. Then there is quibus ea
, followed by the editor's comma, then quibus ea, quae
. quae is a (third) relative pronoun, introducing a relative clause which must have its own subject and verb. Now let's think about Hoyos' Rule 6:
a. Once a subordinate clause or phrase is begun, it must be completed syntactically before the rest of the sentence can proceed.
b. When one subordinate construction embraces another, the embraced one must be completed before the embracing one can proceed.
Is the subordinate clause begun by quae
embraced by the subordinate clause quibus ea
, which would then need its own completion after the quae
clause completes? Clearly not, especially when you read ahead and see that the completion of the quae
clause coincides with the completion of the whole sentence.
Therefore quibus ea
must be a complete clause. What is its subject and verb? The subject is ea
and the verb is mentally supplied by the preceding clause, i.e., sunt
. The antecedent of quibus
is isdem in erratis
, so your mind may also supply an (in
) before quibus
, though that isn't required: (in) quibus ea (sunt)
What does ea
refer to? quae de Platone dicimus
Does that make sense?