Learn Hardcore German: Auf der Einladungskarte steht nur etwas Geheimnisvolles, damit die Überraschungsparty wirklich eine Überraschung bleibt. - On the invitation card there is only something mysterious, so that the surprise party really remains a surprise.

In der Einladungskarte would suggest something inside the card (e.g. inside a folded card, or physically inside an envelope), and is not how you usually talk about what is written on it.

So the logic is: writing is on the surface → auf, not in.

Why is it der Einladungskarte and not die Einladungskarte?

Die Einladungskarte is the basic form (nominative singular), but the preposition auf can take either accusative or dative:

auf
- accusative = movement to a place (onto)
auf
- dative = location (already on something)

Here we are describing where the text is (location, no movement), so auf takes the dative:

feminine noun:
- nominative: die Einladungskarte
- dative: der Einladungskarte

So auf der Einladungskarte is “on the invitation card” in the dative.

What is the subject of the first clause, and why does the sentence start with Auf der Einladungskarte?

The subject of the first clause is etwas Geheimnisvolles.

German main clauses have verb in second position. Position “1” can be many things: subject, time expression, place expression, etc. Here, the place phrase is put in first position for emphasis:

Auf der Einladungskarte (place phrase) → position 1
steht (finite verb) → position 2
nur etwas Geheimnisvolles (subject + adverb) → the rest of the clause

If you make the subject first, you get the more neutral:

Etwas Geheimnisvolles steht auf der Einladungskarte.

Same meaning, different emphasis. Putting Auf der Einladungskarte first focuses on where this mysterious thing is written.

Why is the verb steht used here instead of ist? What does steht mean in this context?

In this context, stehen means “to be written / to be printed” on something. It’s a very common idiomatic use:

Auf der Tafel steht dein Name.
= Your name is written on the board.
In dem Buch steht, dass …
= It says in the book that …

You generally don’t say Auf der Einladungskarte ist etwas Geheimnisvolles for “something mysterious is written on the invitation card.”
Sein (ist) would just say that something exists/“is” there, but stehen carries the idea of standing there as text.

What exactly is etwas Geheimnisvolles, and why does Geheimnisvolles end in -es and start with a capital letter?

Breakdown:

etwas = “something” (an indefinite pronoun, neuter)
geheimnisvoll = “mysterious” (adjective)
Geheimnisvolles here is a nominalized adjective: an adjective used as a noun.

In German, when an adjective is used as a noun (= nominalized), it is:

Capitalized: Geheimnisvolles, not geheimnisvolles
Given an ending according to case, gender, and number.

Here:

etwas is neuter, singular
etwas Geheimnisvolles is the subject of steht, so it’s nominative neuter singular
After pronouns like etwas, nichts, viel, wenig, the adjective takes strong endings
Nominative neuter strong ending is -es

So: etwas Geheimnisvolles literally = something mysterious, grammatically: nominative neuter singular nominalized adjective.

Why is nur placed before etwas Geheimnisvolles and not somewhere else?

Nur means “only” and it usually stands directly in front of the part it limits or focuses on.

Auf der Einladungskarte steht nur etwas Geheimnisvolles.
→ The only thing that stands on the invitation card is something mysterious. (There is nothing else.)

If you move nur, you change what is restricted:

Nur auf der Einladungskarte steht etwas Geheimnisvolles.
→ Only on the invitation card (and nowhere else) something mysterious is written.
Auf der Einladungskarte steht etwas nur Geheimnisvolles.
→ This sounds wrong/unnatural in German.

So nur must go right before etwas Geheimnisvolles to say: “there is only something mysterious (and nothing more) written there.”

What does damit mean here, and how is it different from so dass / sodass or um ... zu?

Damit is a subordinating conjunction that introduces a purpose clause: it answers the question “for what purpose? / so that what?”

In the sentence:

..., damit die Überraschungsparty wirklich eine Überraschung bleibt.
→ “..., so that the surprise party really remains a surprise.”

Differences:

damit = “so that / in order that”
→ expresses intended purpose (someone wants this result)
so dass / sodass = “so that / with the result that”
→ often expresses result or consequence, not necessarily intention
um ... zu = “in order to + verb”
→ only works if the subject of both clauses is the same, and it must go with an infinitive.

Example:

Er schreibt nur etwas Geheimnisvolles auf die Karte, damit die Party eine Überraschung bleibt.
→ He does it with that purpose.
Er schreibt etwas sehr Auffälliges auf die Karte, sodass alle sofort misstrauisch werden.
→ He writes it, and as a result, everyone becomes suspicious.

Here damit is correct because it expresses the intention: They write only something mysterious, in order that the party stays a surprise.

Why does the verb go to the end in damit die Überraschungsparty wirklich eine Überraschung bleibt?

Damit introduces a subordinate clause. In German subordinate clauses:

The finite verb (= conjugated verb) goes to the end of the clause.

So:

main clause: ... die Überraschungsparty bleibt eine Überraschung. (verb in second position)
subordinate clause with damit:
..., damit die Überraschungsparty wirklich eine Überraschung bleibt.

Word order inside the subordinate clause:

damit (subordinating conjunction)
die Überraschungsparty (subject)
wirklich (adverb)
eine Überraschung (predicate noun)
bleibt (finite verb at the end)

That verb-final pattern is standard for clauses introduced by dass, weil, wenn, damit, obwohl, etc.

Why do we say die Überraschungsparty ... eine Überraschung bleibt and repeat Überraschung? Isn’t that redundant?

It is a bit repetitive, but it’s perfectly natural in German. The structure is:

die Überraschungsparty (subject, a specific event)
eine Überraschung (a general “surprise” as a state or property)
bleibt (remains / stays)

So the idea is: “The surprise party remains *a surprise.”
We’re not saying the party stays the same party (that would be trivial), but that its *quality as a surprise is preserved.

You could avoid the repetition:

..., damit es wirklich eine Überraschung bleibt.
(“..., so that it really remains a surprise.”)

Here es refers back to the whole situation (the party). The given sentence just spells out Überraschungsparty again for clarity and emphasis.

How does Überraschungsparty work as a word, and why is it feminine?

Überraschungsparty is a compound noun:

die Überraschung = the surprise
die Party = the party
→ die Überraschungsparty = surprise party

In German compounds, the gender is determined by the last part (the head of the compound). Since Party is feminine (die Party), the whole compound is also feminine:

die Überraschungsparty
der Überraschungsparty (dative/genitive singular)

Both parts are capitalized because they’re nouns, but in writing they form one single word.

Why is there a comma before damit?

German uses a comma to separate main clauses from subordinate clauses.

Main clause: Auf der Einladungskarte steht nur etwas Geheimnisvolles
Subordinate clause introduced by damit:
damit die Überraschungsparty wirklich eine Überraschung bleibt

Since damit starts a full subordinate clause with its own subject (die Überraschungsparty) and verb (bleibt), a comma is required before it:

..., damit die Überraschungsparty wirklich eine Überraschung bleibt.

This is standard punctuation for all clauses introduced by subordinating conjunctions like damit, weil, dass, obwohl, wenn, etc.

Could I also say Es steht nur etwas Geheimnisvolles auf der Einladungskarte? How does that differ from the original?

Yes, you can say:

Es steht nur etwas Geheimnisvolles auf der Einladungskarte.

Differences:

Es here is a dummy subject, similar to English “there” in “There is something written on the card.”
Word order and emphasis:
- Auf der Einladungskarte steht nur etwas Geheimnisvolles.
  → Emphasis on the location (“On the invitation card, there is only something mysterious.”)
- Es steht nur etwas Geheimnisvolles auf der Einladungskarte.
  → More neutral; es fills the subject position, and the place phrase moves to the end.

Both are grammatically correct. Native speakers would probably prefer the original version when they want to highlight what’s (only) on the card.

Why is it bleibt (stays/remains) and not some conditional like würde ... bleiben?

Bleibt is in the present indicative and describes a general intended outcome:

..., damit die Überraschungsparty wirklich eine Überraschung bleibt.
→ “…so that the surprise party really remains a surprise.”

This is a straightforward purpose: We do X so that Y stays true.

Using a conditional like würde bleiben (würde … bleiben) would sound odd here, because there is no hypothetical condition being discussed; it’s a simple goal they want to achieve.

You’d use conditional forms if you were talking about hypothetical or unreal situations, e.g.:

Damit die Party eine Überraschung bliebe, müssten wir vieles geheim halten.
(“So that the party would remain a surprise, we would have to keep many things secret.”)

In the original, they’re just describing what they’re actually doing in reality, so the normal present bleibt is appropriate.