Why "die schlimmsten" and not "am schlimmsten"?

Question

I’m having trouble understanding when to use der/das/die with a superlative and when am. I read here

Use a form of “der/das/die” with the superlative if it is right in front of a noun. Otherwise, use “am” with the superlative

Then, why is it die schlimmsten in the following sentence?

Die serbischen Polizisten waren die schlimmsten?

Should I assume that a noun has been ommited at the end of the sentence (“waren die schlimmsten [Polizisten/Menschen]”)? Is it because in this particular case, the verb is sein?

Answer 1

Your assumption is totally right. The sentence just omitted the noun.

Both (Menschen and Polizisten) are right in that context.

Just to make it even more confusing: The sentence would be totally right if written as Die serbischen Polizisten waren am schlimmsten

Usually the form as chosen in the linked article is only used in a context of direct comparison and usually is not used on its own.

In that example they talk about "Ungarn", "Serbien" and "Mazedonien" and among all of them the serbische Polizisten were the worst. That's why it is written as it is.

Answer 2

You are half (or maybe three quarters ;) ) correct. In your example, there's is an implied noun. And the verb "sein" ("to be") has something to do with it, but in a different way.

You can use the superlative in two different ways: You can use it in an adjectival manner to further describe a noun. Or you can use it in an adverbial manner to further describe a verb.

In the adjectival manner, you have an article in front of the noun that's described further:

Die roten Bonbons sind die leckersten Bonbons.

In English, that would be

The red sweets are the tastiest sweets.

But in most cases you wouldn't repeat the noun, but shorten it in both languages:

Die roten Bonbons sind die leckersten.

The red sweets are the tastiest ones.

If you use the superlative in an adverbial manner, you have to use "am" in German:

Der neue Kollege arbeitet am schnellsten.

The new coworker works the fastest.

If the verb that is described further by your superlative happens to be "sein" ("to be"), the variants are similar and a bit confusing:

Die roten Bonbons schmecken am leckersten.

The red sweets taste the tastiest.

Die roten Bonbons sind am leckersten.

The red sweets are the tastiest.

but, as above

Die roten Bonbons sind die leckersten.

The red sweets are the tastiest ones.

Answer 3

What the other answers fail to spell out: the difference between the forms am schlimmsten and die schlimmsten is that the former is an adverb while the latter is an adjective. Oftentimes in German grammar resources, I have seen the authors make no distinction between the two when they actually should because of the differences.

Die serbischen Polizisten waren die schlimmsten.

In this sentence, there is no implied extra noun. It is merely an example of predicative usage of an adjective. Compare:

Die kroatischen Polizisten waren groß.
Die großen, kroatischen Polizisten …

Die griechischen Polizisten waren größer.
Die größeren, griechischen Polizisten …

Die bulgarischen Polizisten waren die größten.
Die größten, bulgarischen Polizisten …

This contrasts predicative and attributive usage of the corresponding adjectives.

The form with am, being an adverb, initially modifies the verb. However, in the case of sein being the verb, there is hardly any difference between using an adverb with sein or a predicative adjective. Compare:

Die serbischen Polizisten bestrafen am schlimmsten.

Which is not equal to:

Die serbischen Polizisten bestrafen die Schlimmsten.

This last sentence is finally an example where a noun has been left out; one could interpret it both as an ellipsed noun to be filled in with background knowledge or a nominalised adjective. In any case, it is important that in the final sentence die Schlimmsten can no longer refer to the policemen since bestrafen cannot introduce a predicative adjective. Instead die Schlimmsten must be someone else, e.g. die schlimmsten Verbrecher.