Learn Hardcore Swedish: Vi träffar några vänner i centrum. - We are meeting some friends in the city center.

träffa (+ object): to meet someone. Vi träffar några vänner.
träffas (s-verb, reciprocal): to meet each other; no object. Vi träffas i centrum.
möta (+ object) / mötas (reciprocal): to meet/encounter, often while moving toward each other. Jag mötte henne på gatan. Vi möts vid ingången.
ses (reciprocal, very common): to see each other. Vi ses i centrum! Note: träffas med is not idiomatic; just use träffas. Colloquial möta(s) upp exists but many prefer plain möta/mötas or ses.

Why is it i centrum and not på centrum?

With the city center you use i (in): i centrum. Saying på centrum is generally wrong in standard Swedish. Compare:

i centrum, i staden
but idiomatic: på stan (“out in town”), and also på jobbet, på universitetet So: stick with i centrum.

Do I ever use an article with centrum (like “the”)?

Yes, centrum is a neuter noun: ett centrum. In the fixed place phrase you normally drop the article: i centrum. Use the definite when you mean a specific center: i centrumet (e.g., a particular shopping center), or say i stadens centrum.

What exactly does några mean, and how is it used?

några = some / a few (small, unspecified number), used only with plural countable nouns: några vänner, några böcker.
Singular “some/any”: någon (common gender), något (neuter): någon vän, något problem.
With mass nouns, lite is more natural than något: lite vatten (though något vatten is possible, more formal).
Negative: inga (no/any…not), or inte några. Vi har inga pengar ≈ Vi har inte några pengar (the former is more idiomatic).

Why is it vänner and not a regular plural? What are the forms of vän?

vän is irregular:

Singular: en vän
Definite singular: vännen
Plural: vänner
Definite plural: vännerna

Is vänner the usual word for “friends” in everyday speech?

It’s fine, but in casual speech kompisar is even more common. Nuance:

vänner: friends (often a bit more formal/stronger bond)
kompisar: buddies/pals (neutral, everyday)
bekanta: acquaintances
polare: very colloquial

Can I change the word order, like fronting the place: “In the center we meet some friends”?

Yes. Swedish is verb-second. If you front the place, the finite verb must be second:

Neutral: Vi träffar några vänner i centrum.
Fronted place: I centrum träffar vi några vänner. (not: I centrum vi träffar …) With time + place, time usually comes first: I morgon träffar vi några vänner i centrum.

How do I pronounce the tricky words here?

Approximate guidance:

träffar: TRÄF-far [ˈtrɛfːar] (short ä, double f)
några: NÅ-gra [ˈnoːɡra] (long å)
vänner: VÄN-ner [ˈvɛnːɛr] (short ä, double n)
centrum: SEN-trum [ˈsɛntrɵm] (‘c’ like s)

Could träffa mean “to hit,” and is that a problem here?

Yes, träffa can mean “hit/strike (a target)”: Bollen träffade fönstret. With people and this context, Vi träffar några vänner can only mean “meet.” For physically hitting people you’d use slå.

How do I put this in the past or future?

Past: Vi träffade några vänner i centrum.
Planned/intentional future: Vi ska träffa några vänner i centrum.
Neutral prediction: Vi kommer att träffa några vänner i centrum. Present can also express scheduled future if context makes it clear: I kväll träffar vi …

Do I need med (“with”) after träffa, like “träffa med några vänner”?

No. träffa takes a direct object: träffa några vänner. Don’t say träffa med. If it’s reciprocal, use träffas: Vi träffas i centrum.

Are there other common ways to say “downtown/in town” besides i centrum?

Yes:

på stan: very common, “out in town/downtown” in general.
i stan: “in town/in the city,” neutral.
i city: colloquial in some places (esp. Stockholm), not as general as i centrum.