Das Formular, über das wir gestern gesprochen haben, müssen Sie bis morgen abgeben.

Because German often moves an element to the first position for emphasis or topic.

The more neutral word order would be:

Sie müssen das Formular bis morgen abgeben.

By putting Das Formular first, the speaker highlights the form specifically:

Das Formular, über das wir gestern gesprochen haben, müssen Sie bis morgen abgeben.

So the sentence still means the same thing, but the focus is more like:

As for the form we talked about yesterday, you have to hand it in by tomorrow.

Also, in German, whatever comes first counts as position 1, and the finite verb must still stay in position 2. That is why you get müssen Sie instead of Sie müssen.

Because that part is a relative clause.

It gives extra information about das Formular:

• Das Formular = the form
• über das wir gestern gesprochen haben = that we talked about yesterday

German relative clauses are separated by commas, and the finite verb goes to the end of the clause. That is why you see:

über das wir gestern gesprochen haben

with haben at the end.

The two das words do different jobs.

1. Das Formular

   • Here das is the definite article
   • It means the form

2. über das wir gestern gesprochen haben

   • Here das is a relative pronoun
   • It refers back to das Formular

So:

• first das = the
• second das = which/that

English often uses that or leaves it out, but German must use an explicit relative pronoun here.

Because the verb phrase is über etwas sprechen, which means to talk about something.

The preposition über is required by the expression. Since the relative clause refers to das Formular, the relative pronoun has to appear with the same preposition:

• Wir haben über das Formular gesprochen.
• das Formular, über das wir gesprochen haben

So the relative pronoun must match the preposition used with the verb.

The relative pronoun has to match the noun it refers to in gender and number, but its case depends on its role inside the relative clause.

The noun is:

• das Formular = neuter singular

Inside the relative clause, it appears after über, and here über etwas sprechen takes the accusative.

So:

• gender/number: neuter singular
• case: accusative

That gives:

• nominative: das
• accusative: das
• dative: dem
• genitive: dessen

So über das is correct.

Because German subordinate clauses, including relative clauses, send the conjugated verb to the end.

The relative clause is:

über das wir gestern gesprochen haben

This is in the perfect tense, so it contains:

• gesprochen = past participle
• haben = auxiliary verb

In a main clause, you would say:

Wir haben gestern über das Formular gesprochen.

But in the relative clause, the verb material moves to the end:

..., über das wir gestern gesprochen haben, ...

That is standard German word order for subordinate clauses.

Because the whole phrase

Das Formular, über das wir gestern gesprochen haben

comes first in the sentence and fills position 1.

German main clauses follow the verb-second rule. That means the finite verb must come in position 2, no matter what is in position 1.

So the structure is:

1. Das Formular, über das wir gestern gesprochen haben
2. müssen
3. Sie
4. bis morgen
5. abgeben

If the subject came first, then you would have:

Sie müssen das Formular bis morgen abgeben.

Both are correct, but the original sentence emphasizes das Formular.

Because abgeben is a separable verb, and when it appears with a modal verb like müssen, it stays as a full infinitive at the end.

Compare:

• Without a modal:

  • Sie geben das Formular morgen ab.
  • Here it splits: geben ... ab

• With a modal:

  • Sie müssen das Formular morgen abgeben.
  • Here it does not split

So in your sentence:

... müssen Sie bis morgen abgeben.

That is exactly what we expect with a modal verb + separable infinitive.

It is in the accusative, because it is the direct object of abgeben.

You can see this more clearly in the neutral order:

Sie müssen das Formular bis morgen abgeben.

Here:

• Sie = subject
• müssen = finite verb
• das Formular = direct object
• abgeben = infinitive

Since Formular is neuter, nominative and accusative both use das Formular, so the form does not change. But grammatically, it is accusative in the main clause.

Because this is the formal you.

German has:

• du = informal singular
• ihr = informal plural
• Sie = formal singular or plural

In writing, formal Sie is capitalized. So the sentence is addressing someone politely or formally.

An informal version might be:

Das Formular, über das wir gestern gesprochen haben, musst du bis morgen abgeben.

Bis morgen is the normal idiomatic way to say by tomorrow or until tomorrow.

German often uses bis directly with time expressions:

• bis morgen
• bis Montag
• bis nächste Woche

You can sometimes see forms with an article in other contexts, but here bis morgen is the natural everyday phrasing.

Yes. That is also correct.

The difference is mainly focus and style.

1. Original: Das Formular, über das wir gestern gesprochen haben, müssen Sie bis morgen abgeben.

   • emphasizes the form

2. Alternative: Sie müssen das Formular, über das wir gestern gesprochen haben, bis morgen abgeben.

   • starts more neutrally with the subject

Both are grammatical. German is flexible with word order, but the finite verb in the main clause still has to stay in second position.

German often uses relative clauses exactly where English would also use that or which clauses:

• the form that we talked about yesterday

In German:

• das Formular, über das wir gestern gesprochen haben

You could sometimes rephrase, but this version is very normal and natural. It is a standard way to attach extra information to a noun.

Not in this sentence.

Worüber is used when there is no noun directly before it to refer back to, for example:

• Worüber haben wir gestern gesprochen?
  • What did we talk about yesterday?

But when you are referring back to a noun like das Formular, German uses a relative pronoun:

• das Formular, über das wir gestern gesprochen haben

So:

• with a noun antecedent: über das
• without a noun antecedent: worüber

It is the perfect tense:

• sprechen → gesprochen
• auxiliary: haben

So:

wir haben gestern gesprochen = we spoke yesterday / we have talked yesterday

In everyday spoken German, the perfect tense is very common for past events, especially in conversation. That is why gesprochen haben sounds natural here.

A Präteritum version would be possible in some contexts:

Das Formular, über das wir gestern sprachen, müssen Sie bis morgen abgeben.

But that sounds more literary or formal than the original.