Wir treffen uns um halb neun im Park.

Because they do different jobs:

• wir = the subject of the sentence, the people doing the action
• uns = a reflexive pronoun, used with sich treffen

In German, sich treffen is the usual way to say to meet in the sense of meeting up.

So:

• Wir treffen uns. = We meet / We’re meeting up.

English does not use a reflexive pronoun here, but German usually does.

German main clauses usually follow the verb-second rule.

That means the finite verb must come in the second position:

• Wir treffen uns ...

Here:

• Position 1: Wir
• Position 2: treffen
• then: uns

So uns treffen would sound wrong in a normal statement like this.

Sich treffen is a very common German verb phrase meaning to meet or to meet up.

It changes depending on the subject:

• ich treffe mich
• du triffst dich
• wir treffen uns
• ihr trefft euch
• sie/Sie treffen sich

In this sentence, wir treffen uns means we are meeting each other / we’re meeting up.

Because the reflexive pronoun changes with the person.

For wir, the correct reflexive pronoun is uns.

Compare:

• Ich treffe mich
• Du triffst dich
• Er trifft sich
• Wir treffen uns
• Ihr trefft euch
• Sie treffen sich

So sich treffen is the dictionary form, but in a real sentence you use the matching form like mich, dich, uns, etc.

Um is the usual preposition used before a specific clock time.

Examples:

• um acht Uhr
• um Viertel nach zwei
• um halb neun

So in this sentence, um halb neun means at half past / at 8:30 in English terms.

This is one of the most important time expressions for English speakers to learn.

In German, halb neun literally means half to nine, not half past nine.

So:

• halb acht = 7:30
• halb neun = 8:30
• halb zehn = 9:30

English speakers often misinterpret this at first, because English half nine can mean different things depending on the variety of English, and American English usually says 8:30 instead.

Because German nouns normally need an article in this kind of sentence.

im is a contraction of:

• in dem → im

So:

• im Park = in the park

You usually cannot say just in Park here.

It is in the dative case.

That is because in can take either:

• accusative for movement toward somewhere
• dative for location

Here the sentence describes where the meeting happens, not movement into the park.

So:

• im Park = location → dative

Compare:

• Wir treffen uns im Park. = We meet in the park.
• Wir gehen in den Park. = We go into the park.

Yes. That is completely natural.

German word order is more flexible than English, especially with time and place phrases.

Both are fine:

• Wir treffen uns um halb neun im Park.
• Wir treffen uns im Park um halb neun.

A common guideline is TMP:

• Time
• Manner
• Place

So time before place is often the default, which is why um halb neun im Park sounds especially standard here. But both orders are correct.

Because German often uses the present tense for scheduled or planned future events, just like English often does.

So:

• Wir treffen uns morgen.
• Wir treffen uns um halb neun im Park.

These can refer to the future without needing a special future tense.

German does have a future form, but in everyday speech the present tense is very common when the time is clear.

No, treffen is not separable.

The sentence has two parts, treffen and uns, but that does not make it a separable verb. The uns is a reflexive pronoun, not a verb prefix.

So:

• Wir treffen uns. = reflexive verb construction
• not a separable verb like anrufen → Ich rufe an

A rough guide:

• Wir → like veer
• treffen → TREFF-en
• uns → oons with a short vowel
• um → oom
• halb → sounds like halp at the end, because final b is devoiced
• neun → like noyn
• im → im
• Park → park, but with a German r

A natural rhythm would be:

Wir TREFFen uns um halb NEUN im PARK.

Normally, no, not with this meaning.

Wir treffen um halb neun im Park would sound incomplete or wrong to most speakers, because treffen usually needs either:

• an object: Wir treffen ihn im Park. = We meet him in the park.
• or the reflexive form: Wir treffen uns im Park. = We meet each other / we meet up in the park.

So uns is necessary here.