"Everything he said, was true."
Translation:Alles was er sagte, war richtig.
I seems to me that Magister_Smith may have incorrectly used "das" instead of "dass" in his question (since "dass" rather than "das" is used to introduce a subordinate clause).
So let me try asking again: why is "Alles, dass er sagte, war richtig" is incorrect?
For comparison, in English you cannot say "everything what he said, was true", it would have to be "everything that he said ...". So why is it suddenly "was" instead of "dass" in German (given that otherwise they are very strongly correlated).
"Dass" wouldn't make sense here at all. We're using a relative pronoun here, so we use a "der/die/das" pronoun, not "dass." (If the subject were, for instance, "die Worte," we would say "Die Worte, die er sagte," not "Die Worte, dass er sagte.")
The reason for "was" instead of "das" is that many indefinite pronouns ("etwas," "nichts," "alles," and some others) simply use "was" as their relative pronoun. I think this is because these words don't technically have a gender, so using the neuter "das" for them wouldn't make sense; instead a genderless pronoun "was" is used.
I believe that "das" is completely correct. "Das" and "was" both work as relative pronouns.
EDIT: Nope, I'm wrong. We use "was" instead of "das" if (and only if) the antecedent is an indefinite pronoun. (http://www.lsa.umich.edu/german/hmr/Grammatik/Relativsaetze/relative.html)
Do you mean a German native speaker? The reasoning is actually the same in both languages, as far as I can tell, but I will explain in English.
The main verb of the sentence is "was".
The sentence has two verbs in it because one is in a relative clause with an omitted relative pronoun. The full sentence should read: "Everything THAT he said, was true". So the main clause is "Everything was true," and the relative clause reads "that he said." That is why we see the comma there.
That's just not a grammatical sentence and doesn't make any sense.
The structure here uses a relative clause, though the English sentence kind of obfuscates it. You can say "Everything that he said was true" to make it stand out. Then the phrasing is more clear: "Alles, was er sagte, stimmte."