Where is the subject in this sentence?

Question

I found this sentence on the first page of an A1 German text for language learners, and I'm stumped. I was hoping someone could explain the grammar of this sentence?

Ihn interessiert nur, was mit Verbrechen zu tun hat.

I understand that the second half is a dependent clause meaning something like "whatever has to do with breaking the law." But I don't understand the first half of this sentence. Where is the subject?

For context, the previous sentence reads:

Die meisten Mädchen finden ihn hübsch, aber das ist ihm egal.

Both of these sentences are about the main character Georg.

Answer 1

Ihn interessiert nur, was mit Verbrechen zu tun hat.

That dependent clause you have here is a so called subject clause. Yep. That clause is the subject.

You could add an expletive das but it's still the dependent clause that is the subject.

Ihn interessiert nur das, was mit Verbrechen zu tun hat.

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FYI, unlike Enlish clauses, German clauses don't require a subject. You can see this best with impersonal passive voice:

Er arbeitet hart.

This sentence has no accusative object, so the only way to build a passive voice from it is without a subject:

Von ihm wird hart gearbeitet.

Again, adding an expletive is possible but the sentence is still without a subject:

Es wird von ihm hart gearbeitet.

Answer 2

A number of difficulties come together in the example: there's a clausal subject, and this is a free relative clause, and it occurs at the very end of the whole sentence, and there's no pronoun inside the clause that announces it (because that's hardly possible with free relatives, as opposed to "that"-clauses).

Here, the preference to put subordinate clauses at the end overrides the preference to put the subject first.

But it would be possible to have the relative clause at the start: [Nur was mit Verbrechen zu tun hat] interessiert ihn wirklich. I feel that the original version is slightly smoother stylistically, but it's hard to say why.