Learn Hardcore German: Ich frage, wen wir morgen im Park treffen. - I ask whom we will meet in the park tomorrow.

The part wen wir morgen im Park treffen is an indirect question clause (a type of subordinate clause) introduced by the question word wen.

In German subordinate clauses, the conjugated verb goes to the end of the clause. So:

Direct question: Wen treffen wir morgen im Park?
(Verb in second position)
Indirect question: … wen wir morgen im Park treffen.
(Same words, but now a subordinate clause → treffen goes to the end)

So the word order changes because the question is embedded inside another sentence (after Ich frage, …), and embedded questions follow the normal subordinate-clause word order: [… Subject – other elements – Verb].

Is wen wir morgen im Park treffen itself a question?

Grammatically, it is not a direct question, but an indirect question (also called an embedded question).

Compare:

Direct question:
Wen treffen wir morgen im Park? – This is a full question on its own.
Indirect question embedded in a statement:
Ich frage, wen wir morgen im Park treffen. – The whole sentence is a statement about a question you are asking.

In other words, you’re not asking the person you’re speaking to directly; you’re stating that you are asking (or wondering) about something. The embedded clause still has a question word (wen) but obeys subordinate clause word order (verb at the end).

Why is there a comma before wen?

In German, commas are mandatory between:

a main clause and
a subordinate clause

Here:

Ich frage = main clause
wen wir morgen im Park treffen = subordinate clause (indirect question)

Therefore German must have a comma:

✅ Ich frage, wen wir morgen im Park treffen.

Leaving out the comma here is considered incorrect in standard written German.

Could I also say Ich frage, wer wir morgen im Park treffen?

No, not with the same meaning. Wer and wen change who does what to whom.

wen wir treffen = whom we meet (we are the subject, the other person is the object)
wer uns trifft = who meets us (the other person is the subject)

Your sentence means I ask whom we will meet tomorrow in the park, so:

Subject = wir
Object (person being met) = wen

So Ich frage, wen wir morgen im Park treffen is correct.
Ich frage, wer wir morgen im Park trifft would be ungrammatical even after fixing the verb ending (trifft), because wer can’t be the subject of wir trifft. To make wer the subject, you’d need something like:

Ich frage, wer uns morgen im Park trifft. – I ask who will meet us tomorrow in the park.

That’s a completely different relationship between subject and object.

Why is it Ich frage and not Ich frage mich if the meaning is “I wonder…”?

German has two slightly different constructions:

Ich frage …
Literally: I ask …
This usually implies you ask someone (maybe not stated) a question.
- Ich frage, wen wir morgen im Park treffen.
  Implies you ask (someone) this question.
Ich frage mich …
Literally: I ask myself …
This is the usual way to express “I wonder …” in German.
- Ich frage mich, wen wir morgen im Park treffen.
  = I wonder whom we will meet tomorrow in the park.

Both are grammatically correct, but they’re not identical in nuance:

Ich frage, … = I am asking (someone a question).
Ich frage mich, … = I am wondering / I’m asking myself.

Why is it im Park and not in dem Park or am Park?

im is simply the contracted form of in dem:

in dem Park → im Park

Both mean “in the park”, and im Park is the natural, everyday form.

Why in and not an?

in = inside something, within its boundaries → im Park = inside the park
an = at, by, next to (often with edges, surfaces, rivers, etc.)
am Park (an dem Park) would mean by the park / next to the park, not inside it.

So your sentence talks about meeting inside the park, so im Park is the right choice.

Is the time word morgen correct without a preposition like am?

Yes. Morgen can function directly as an adverb of time:

Wir treffen ihn morgen. – We’ll meet him tomorrow.

You only use am with Morgen when you mean “morning” (the time of day), not “tomorrow”:

morgen (lowercase) = tomorrow
der Morgen (noun) = morning
→ am Morgen = in the morning

So:

Ich frage, wen wir morgen im Park treffen. – tomorrow (correct)
Ich frage, wen wir am Morgen im Park treffen. – in the morning (also correct, but different meaning)

Could I say Ich frage, wen wir morgen treffen im Park with im Park at the end?

You can put im Park at the end, and you will be understood:

Ich frage, wen wir morgen treffen im Park.

However, this sounds a bit clumsy or spoken/colloquial. The more natural, neutral word order for this subordinate clause is:

… wen wir morgen im Park treffen.

German tends to prefer the order Time – Place – Verb (and keeps heavier elements away from the very end when possible). So morgen im Park treffen feels smoother than morgen treffen im Park in this particular sentence.

Why isn’t future tense used, like treffen werden, since it’s about tomorrow?

German frequently uses the present tense to talk about future actions, especially when a time expression (like morgen) makes the time clear.

So both of these are correct:

Ich frage, wen wir morgen im Park treffen.
Ich frage, wen wir morgen im Park treffen werden.

Differences:

… treffen. = neutral, very common in everyday speech and writing.
… treffen werden. = slightly more explicit / formal; emphasizes the future aspect more, but often not necessary.

In normal conversation, the version with present tense + time word is usually preferred.