Learn Hardcore German: Das Formular ist bis morgen auszufüllen. - The form is to be filled out by tomorrow.

It’s the sein + zu + infinitive construction: ist … auszufüllen. This behaves like an impersonal, instruction-like passive and means roughly “is to be filled out” or “must be filled out.”

How does sein + zu + infinitive differ from müssen + Passiv (e.g., muss … ausgefüllt werden)?

They’re near-equivalents in meaning:

Das Formular ist … auszufüllen = instruction/formal requirement.
Das Formular muss … ausgefüllt werden = explicit obligation (“must … be filled out”). The sein + zu form sounds more official and impersonal; the müssen passive is more neutral and common in everyday speech.

Why is it ist and not wird?

Because this is not the normal passive with werden. Sein + zu + infinitive is its own pattern of expressing necessity. Das Formular wird bis morgen ausgefüllt would mean “The form is being filled out by tomorrow,” which is odd; to express a future passive, you’d say wird … ausgefüllt werden, but that talks about a future process, not an instruction or requirement.

Where does zu go with separable verbs like ausfüllen?

With separable verbs, zu is placed between the prefix and the stem: aus|zu|füllen. Examples:

auf|zu|machen (to open)
an|zu|rufen (to call)
Not: ✗ zu ausfüllen or ✗ auszufüllen with the parts separated.

Is auszufüllen a participle?

No. Auszufüllen is a zu-infinitive. The past participle is ausgefüllt. Compare:

Das Formular ist auszufüllen. = is to be filled out (requirement)
Das Formular ist ausgefüllt. = is filled out (resulting state)

Who is the subject here, and who is doing the action?

The grammatical subject is Das Formular. The doer (agent) is left unspecified—this construction intentionally avoids naming who must do it (understood as “someone/you/we” in context).

How would I say this directly to a person?

Polite: Bitte füllen Sie das Formular bis morgen aus.
Informal: Du musst das Formular bis morgen ausfüllen. These make the agent explicit and sound more conversational.

Can I name who is responsible in this construction?

Yes, with von + Dativ, though it’s formal: Das Formular ist von dir/Ihnen bis morgen auszufüllen. In most contexts, using an active sentence (e.g., Sie müssen …) is clearer and more natural.

Where should bis morgen go in the sentence?

The most natural is where you saw it: Das Formular ist bis morgen auszufüllen.
You can front it for emphasis: Bis morgen ist das Formular auszufüllen.
Das Formular ist auszufüllen bis morgen is possible but sounds less idiomatic; German tends to place time adverbials before the final non-finite verb cluster here.

What case does bis take here?

Bis governs the accusative. With bare time words you won’t see an article:

bis morgen, bis nächste Woche, bis Mitternacht. With nouns that take an article, you’ll see it clearly:
bis nächsten Montag (accusative masculine strong ending -en)
bis den 15. Mai

Is there a difference between bis morgen and bis zum Morgen?

Yes:

bis morgen = by tomorrow (deadline).
bis zum Morgen = until the morning (of a day), focusing on the time-of-day period; it’s not how you’d normally set a deadline for the next day. For a deadline, use bis morgen.

Does this construction always express obligation? Can it mean possibility?

Both exist:

Obligation/requirement: Die Aufgabe ist heute zu erledigen.
Possibility (often with an adverb/adjective): Die Tür ist leicht zu öffnen = “can be opened easily.” In your sentence, the deadline bis morgen clearly points to obligation.

What tense is this? It mentions tomorrow but uses the present.

It’s the present tense. German often uses the present to talk about future deadlines or scheduled events. The deadline is conveyed by bis morgen.

Can I say Das Formular ist bis morgen ausgefüllt?

That describes a state (“The form is filled out by tomorrow”), i.e., a prediction about the result by tomorrow. It’s not an instruction. To give an instruction/requirement, use ist … auszufüllen or muss … ausgefüllt werden.

Could I use sollen instead?

Yes: Das Formular soll bis morgen ausgefüllt werden. This means “is supposed to be filled out by tomorrow,” which can sound slightly weaker or more like a guideline than a strict obligation.

What’s the register of sein + zu + Infinitiv?

Formal/official. It’s common in written instructions, regulations, and notices, and less common in everyday conversation.

How do I pronounce auszufüllen?

Stress the prefix in ausfüllen: primary stress on aus.
zu is [tsu], unstressed here.
ü in füllen is the fronted vowel [y] (like the French “u” in “tu”).
Final -en is typically a reduced sound [ən]. So roughly: [AUS-tsu-FY-lən].

What are the gender and plural of Formular?

It’s neuter: das Formular. The plural is die Formulare.