Learn Hardcore German: Die Stadt teilt mit, der Hof sei morgen geöffnet. - The city announces that the courtyard will be open tomorrow.

It comes from the separable-prefix verb mitteilen “to inform/announce.” In main clauses, separable prefixes move to the end, so you get Die Stadt teilt … mit. Tense examples:

Present: Die Stadt teilt mit …
Simple past: Die Stadt teilte mit …
Perfect: Die Stadt hat mitgeteilt … (here the prefix reattaches)

Why is there a comma after mit? Could it be a colon instead?

The comma separates the main clause from the following content clause in indirect speech. A colon is also possible and is common in official or press style:

With comma (indirect speech): Die Stadt teilt mit, der Hof sei morgen geöffnet.
With colon (often direct speech, but also usable before indirect): Die Stadt teilt mit: Der Hof sei morgen geöffnet. / Die Stadt teilt mit: Der Hof ist morgen geöffnet.

Why is it sei and not ist?

Sei is the Subjunctive I (Konjunktiv I) of sein, used for reported/indirect speech to mark distance: the writer reports what the city says without vouching for it. Using ist would sound like the writer is asserting the fact directly.

What exactly is sei? How is sein conjugated in Konjunktiv I?

It’s the Konjunktiv I present of sein:

ich sei
du seiest
er/sie/es sei
wir seien
ihr seiet
sie/Sie seien

Could I use wäre instead of sei?

Wäre is Konjunktiv II. In reported speech, German prefers Konjunktiv I if it’s distinct from the indicative. Since sei differs from ist, the standard form here is sei. Wäre would either sound colloquial or suggest hypothetical/irrealis, not neutral reporting.

Why is there no dass? Can I add it, and where would the verb go?

Yes, you can add dass:

Without dass (that-less indirect speech, verb in 2nd position): …, der Hof sei morgen geöffnet.
With dass (subordinate clause, verb-final): …, dass der Hof morgen geöffnet sei. Everyday style often uses indicative under dass: …, dass der Hof morgen geöffnet ist. Formal reporting keeps sei.

Why is the verb in the reported part in second position instead of at the end?

Because this is “that-less” indirect speech (an unintroduced content clause). Without a subordinator like dass, the clause keeps main-clause word order (verb in 2nd position): der Hof sei …. If you use dass, the finite verb moves to the end.

Why geöffnet and not offen?

Both can mean “open,” but:

geöffnet is the standard choice for opening hours/being open for business: Der Hof ist (morgen) geöffnet.
offen is the general adjective “open” (a door, a box, an issue): Die Tür ist offen. You can say Der Hof ist offen, but for business/official opening, geöffnet is idiomatic.

What’s the difference between ist morgen geöffnet and wird morgen geöffnet?

ist morgen geöffnet describes the state: it will be open for business tomorrow.
wird morgen geöffnet is eventive/passive: it will be opened tomorrow (someone will open it tomorrow). German usually prefers the present with a time adverbial for future states: ist morgen geöffnet. A true future state can be expressed as wird morgen geöffnet sein, but that’s rarely necessary here.

Does morgen mean “tomorrow” or “in the morning” here, and why is it lowercase?

Here morgen means “tomorrow,” an adverb, so it’s lowercase. The noun der Morgen (“morning”) is capitalized and typically appears with a preposition: am Morgen = “in the morning.” Compare:

Der Hof ist morgen geöffnet. = “The yard is open tomorrow.”
Der Hof ist am Morgen geöffnet. = “The yard is open in the morning.”

What exactly does Hof mean in this context?

Hof is polysemous: “courtyard/yard,” “farm,” “(royal) court,” etc. In municipal announcements, it often refers to a specific local facility like a Wertstoffhof (recycling yard) or Bauhof (municipal yard), or to a courtyard previously mentioned. A law court is typically das Gericht, not der Hof (except in certain official names like Gerichtshof).

Why is it der Hof and not das Hof?

Because Hof is a masculine noun: der Hof (Nom. sg.). In the sentence, it’s the subject of the reported clause, so nominative is used.

Does mitteilen need a person (dative)? Who is being informed here?

The full pattern is jemandem (Dat.) etwas (Akk./content clause) mitteilen. The addressee can be omitted when it’s general/obvious. Here, the content is the clause (der Hof sei …), and the addressee is implicitly the public. You could expand it: Die Stadt teilt den Bürgerinnen und Bürgern mit, dass …

Is this formal style? How would I say it more casually?

Yes, this is official/news style. Neutral everyday alternatives:

Die Stadt sagt, dass der Hof morgen geöffnet ist.
Laut Stadt ist der Hof morgen geöffnet.
Die Stadt gibt bekannt, dass der Hof morgen geöffnet ist. Keeping sei under dass is also fine in formal writing: … dass der Hof morgen geöffnet sei.

Can I move morgen to the front? Is Der Hof sei geöffnet morgen okay?

Morgen sei der Hof geöffnet. is fine (fronting time for emphasis).
Der Hof sei morgen geöffnet. is the neutral variant.
Der Hof sei geöffnet morgen. sounds odd in standard German; time adverbs usually come earlier in the middle field.