Learn Hardcore German: In dieser Stadt ist es verboten, Abfall im Park liegen zu lassen. - In this city it is forbidden to leave trash lying in the park.

German needs a dummy subject here, just like English uses it in it is forbidden. The es doesn’t refer to anything concrete; it’s an expletive subject. If you make the infinitive clause the subject, you drop es:

Abfall im Park liegen zu lassen ist verboten.

Why does the sentence start with In dieser Stadt, and why does the verb come right after it?

German main clauses are verb-second (V2). Only one constituent can come before the finite verb. Here, In dieser Stadt occupies the first slot, so ist must be second, and es follows:

In dieser Stadt ist es verboten, … You could also say:
Es ist in dieser Stadt verboten, …
In dieser Stadt ist Abfall im Park liegen zu lassen verboten. (heavier, less natural)

Why is there a comma before Abfall im Park liegen zu lassen?

It’s an infinitive clause with zu that’s explained/announced by the correlative es and depends on the predicate adjective verboten. In such cases, the comma is required.

What exactly is verboten?

It’s the past participle of verbieten used as a predicate adjective meaning forbidden/not allowed. German often uses a form like ist + participle as a predicative adjective: Es ist erlaubt/verboten/möglich/schwer, …

Why is it im Park and not something else? What case is that?

Im is the contraction of in dem. With location (no movement), in takes the dative: in dem Park → im Park. If it were movement into the park, you’d use accusative: in den Park.

Why dieser Stadt and not diese Stadt?

Stadt is feminine. After in with location, you need dative singular feminine, which is dieser. So: in dieser Stadt. Nominative feminine would be diese Stadt.

Why is there no article before Abfall?

Here Abfall is used generically as a mass/uncountable noun, so no article is needed. Variations:

Specific, known waste: den Abfall
Explicit negation with a different structure: Man darf keinen Abfall im Park liegen lassen.

What’s the difference between Abfall and Müll?

Both mean trash/garbage. Abfall is more formal/administrative (you’ll see it on signs and in rules). Müll is more colloquial. Plurals: Abfälle (types/pieces of waste) exists; Müll rarely has a plural.

What does liegen lassen literally mean, and how does it work?

Literally: to leave (something) lying → to leave something where it is, not pick it up. It’s lassen plus another infinitive (liegen).

Finite example: Er lässt den Müll liegen. (He leaves the trash lying there.)
Past: Er hat den Müll liegen lassen.

Why is it liegen zu lassen and not zu liegen lassen?

With lassen (and perception verbs like sehen/hören), the zu goes with the second infinitive: liegen zu lassen, stehen zu lassen, arbeiten zu lassen. So the correct cluster here is … liegen zu lassen.

Is zu lassen the same as the verb zulassen?

No. Here, zu marks the infinitive, and lassen is its own verb: … liegen zu lassen. The single-word verb zulassen means to allow/permit and is unrelated in this context. Don’t write zulassen here.

Could I front the infinitive clause and drop es: Abfall im Park liegen zu lassen ist verboten?

Yes. Then the infinitive clause itself serves as the subject, so es is unnecessary. This word order is correct and slightly more formal or emphatic.

Could I say the same thing with dürfen?

Yes, a very natural paraphrase is:

In dieser Stadt darf man keinen Abfall im Park liegen lassen. This uses modal negation with dürfen nicht and kein.

Can I swap the order of Abfall and im Park inside the infinitive clause?

Both are possible:

…, Abfall im Park liegen zu lassen. (neutral)
…, im Park Abfall liegen zu lassen. (slight emphasis on location) German allows some flexibility; short/known info often comes earlier.

Why not just say Müll im Park liegen zu lassen or Abfälle?

You can. Nuance:

Abfall (mass/generic) is the most sign-like, formal choice.
Müll is more everyday.
Abfälle suggests multiple items/types of waste and is less common in a general prohibition.

Why present tense ist? Is this about now only?

German present tense covers general, timeless rules and regulations. Ist verboten expresses a standing rule, not just a momentary situation.