Neuter Relative Pronouns: das or was?

Putting it in the simplest possible terms, a relative pronoun is a specific word in a sentence that has a relative clause. It's much easier to understand when you see an example:

The book that I read is quite old.

This sentence is dependent upon "that I read" because without these words, it would not be clear which book is meant. The word "that" is the relative pronoun in the sentence.

In the German language, the relative pronoun is dependent upon the gender of the subject noun:

Das Buch, das ich gelesen habe, ist ganz alt.
Der Mann, den ich gesehen habe, war ganz alt.
Die Frau, die ich gesehen habe, war ganz alt.

As you can see, the definite articles in the nominative case must take on the accusative case as relative pronouns: das/das, der/den, die/die.

But in the case of neuter nominatives, the German word was (usually translated as "what") is also used as a relative pronoun. The use of was as a relative pronoun is generally restricted to two usages, one of which is for neuter substantivized superlatives (nouns based upon adjectives), such as das Beste or das Schönste:

Das Schönste, was ich gelesen habe, war ein Buch von Goethe.

The German word was is also used as a relative pronoun with neuter demonstrative and indefinite pronouns, such as das, dasjenige, dasselbe; alles, einiges, nichts, vieles, manches, weniges, etwas, and so forth.

Das, was Sie hören, ist Musik von Mozart.
Es gibt einiges, was ich noch lernen sollte.

It is incorrect to use the relative pronoun das in the three examples above.

Further Learning
Here are some examples featuring relative pronouns on Yabla German. See if you can fill in the missing relative pronoun with either das or was:

Gab's etwas, nicht so gut war?

Was there something that wasn't so good?

Caption 30, Deutschkurs in Tübingen - Weil oder obwohl? - Part 2
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Das Mädchen, am Spielfeldrand niedlich zu den Jungs hinsah...

The girl who, on the edge of the playing field, looked sweetly at the boys

Captions 2-3, Olli Schulz - Spielerfrau
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Als wäre das Leben, hier einmal war, verbraucht.

As if the life that once was here were used up

Caption 8, Christina Stürmer - Millionen Lichter
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Er ärgert sich auch über manches, über ihn geschrieben wird.

he also gets angry about some of what is written about him.

Caption 19, Thomas D - Ärgernisse
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Aspirin ist ein Medikament, ich nehme, wenn ich Kopfschmerzen habe.

Aspirin is a medication that I take if I have a headache.

Captions 13-14, Deutschkurs in Blaubeuren - Der Relativsatz - Part 16
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Das ist das Beste, es gibt auf der Welt.

That's the best thing that there is in the world

Caption 36, Monsters of Liedermaching - Ein Pferd
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Milch ist ein Getränk, ich nicht mag.

Milk is a drink that I do not like.

Caption 29, Deutschkurs in Blaubeuren - Der Relativsatz - Part 16
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Alles, mit Kommunikation und Sprache zu tun hat.

Everything that has to do with communication and language.

Caption 26, Anja Polzer - Interview - Part 1
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Deinen Namen zu nennen ist wohl das Schönste, ich sage.

Naming your name is absolutely the most beautiful thing that I say

Caption 35, Xavier Naidoo - Ich kenne nichts (das so schön ist wie du)
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Es gibt am Flughafen wohl nichts, es nicht gibt.

Indeed, there's nothing that you won't find at the airport.

Caption 42, Flugreisen - Was mache ich, wenn...
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Click on the video links to see if your choices were correct!

Don't feel bad if this seems hard—even native speakers sometimes get it wrong by accident or as slang usage. The full title of the song above by Xavier Naidoo is "Ich kenne nichts (das so schön ist wie du)." According to grammar rules, the das should have been was. It's also a common mistake among native speakers to say or write things like Das Buch, was ich gelesen habe and Das Buch, dass ich gelesen habe. Luckily, we now know the correct way to write it!