Der Vertrag legt die Endreinigung nach der Abbauzeit fest.

legt ... fest is the present-tense form of the separable verb festlegen.

In this sentence, festlegen means something like:

• to specify
• to determine
• to lay down
• to stipulate

So Der Vertrag legt die Endreinigung ... fest means that the contract specifies/stipulates the final cleaning.

This is a very common verb in formal or legal-style German.

Because festlegen is a separable verb.

In a normal main clause, German splits many separable verbs into:

• the conjugated base verb in position 2: legt
• the prefix at the end: fest

So:

• festlegen → legt ... fest

That is why the sentence is not Der Vertrag festlegt ..., but:

• Der Vertrag legt die Endreinigung nach der Abbauzeit fest.

In an infinitive or subordinate clause, it would stay together:

• Der Vertrag will die Endreinigung festlegen.
• ..., weil der Vertrag die Endreinigung nach der Abbauzeit festlegt.

Der Vertrag is the subject of the sentence, so it is in the nominative case.

The noun Vertrag is masculine:

• nominative: der Vertrag
• accusative: den Vertrag
• dative: dem Vertrag

Here, the contract is the thing doing the action of laying down / specifying, so nominative is required:

• Der Vertrag legt ...

die Endreinigung is the direct object of the verb festlegen, so it is in the accusative case.

The noun Endreinigung is feminine, and for feminine nouns:

• nominative: die
• accusative: die

So the form stays the same:

• nominative: die Endreinigung
• accusative: die Endreinigung

That can be confusing for English speakers, because the article does not visibly change here, even though the case has changed.

Endreinigung is a compound noun:

• End- = final / end
• Reinigung = cleaning

So it means final cleaning or end-of-use cleaning.

In context, it often refers to the cleaning that must happen after an event, rental, installation, or dismantling period.

German uses compound nouns very freely, so long words like this are normal.

Because the preposition nach takes the dative case in this meaning.

So:

• nominative: die Abbauzeit
• dative: der Abbauzeit

That gives:

• nach der Abbauzeit = after the dismantling period

This is a very useful rule to remember:

• nach
  • dative

Examples:

• nach dem Essen = after the meal
• nach der Arbeit = after work
• nach dem Konzert = after the concert

Abbauzeit is another compound noun:

• Abbau = dismantling / taking down / removal
• Zeit = time / period

So Abbauzeit means the dismantling period or the time for taking something down.

This is the kind of noun often used in event, rental, exhibition, or contract language.

For example, if equipment, booths, or structures have to be removed after use, that removal period could be called the Abbauzeit.

Yes. The sentence follows a very typical German main-clause pattern:

• Der Vertrag = subject
• legt = conjugated verb in position 2
• die Endreinigung = object
• nach der Abbauzeit = prepositional phrase
• fest = separable prefix at the end

So the structure is completely natural.

German often places time-related phrases in different positions depending on emphasis, though. For example, you could also say:

• Der Vertrag legt nach der Abbauzeit die Endreinigung fest.

That is grammatically possible, but the original version sounds very natural and clear.

Not really in natural English.

Even though the parts are:

• legen = to lay / put
• fest = firm / fixed

the combined verb festlegen is an idiomatic unit. In real usage, it means:

• to determine
• to specify
• to set
• to stipulate

So it is better to learn festlegen as one vocabulary item, not by translating each part separately every time.

Yes, it sounds fairly formal and somewhat contract-like or administrative.

Words and structures that make it sound formal include:

• Der Vertrag = the contract
• festlegen = to stipulate / specify
• compound nouns like Endreinigung and Abbauzeit

In everyday conversation, someone might express the idea more simply, but in written rules, agreements, and official documents, this style is very common.

Yes, but the nuance would change slightly.

Some possible alternatives are:

• bestimmen = to determine
• regeln = to regulate / set out
• vorschreiben = to prescribe / require

For example:

• Der Vertrag bestimmt die Endreinigung nach der Abbauzeit.
• Der Vertrag regelt die Endreinigung nach der Abbauzeit.

These are similar, but festlegen is especially common when something is being clearly specified or fixed in a contract.

So in this sentence, festlegen is a very natural choice.