In diesem Stadtteil sind die Leute freundlich, und die Atmosphäre ist ruhig.

Because in here answers the question where?, not where to?. In German, in takes:

• dative for location: in diesem Stadtteil
• accusative for movement into something: in diesen Stadtteil

Since the sentence describes a location, you need the dative.

Also, Stadtteil is masculine: der Stadtteil.
So the dative form of dieser is diesem.

German main clauses usually follow the verb-second rule. That means the conjugated verb must come in the second position.

Here, the sentence starts with In diesem Stadtteil, so that whole phrase takes the first position. The verb then has to come next:

• In diesem Stadtteil | sind | die Leute freundlich

If you start with the subject instead, that also works:

• Die Leute sind in diesem Stadtteil freundlich, und die Atmosphäre ist ruhig.

The meaning is very similar, but starting with In diesem Stadtteil puts more emphasis on the location.

No. im is a contraction of in dem.

• in dem → im
• in diesem cannot contract

So:

• im Stadtteil = in the district
• in diesem Stadtteil = in this district

Those are different meanings.

Here die Leute means the people there / the people in that area as a known group.

German often uses the definite article where English might not. In this sentence, die Leute sounds natural because it refers to the people of that neighborhood in general.

If you said just Leute, it would usually sound less complete or would change the meaning.

So:

• die Leute = the people there, the people in that district

Leute and Menschen can both mean people, but they are not always identical.

• Leute is very common and everyday
• Menschen is a bit broader and sometimes more formal or abstract

In a sentence about a neighborhood, die Leute sounds very natural.

Also, Leute is normally used only in the plural. There is no normal singular form meaning one person.

You would not usually say ein Leute.
For one person, you would say eine Person, ein Mensch, ein Mann, eine Frau, etc., depending on context.

Because after sein, adjectives are used predicatively, and predicative adjectives do not take endings in German.

So you say:

• Die Leute sind freundlich.
• Die Atmosphäre ist ruhig.

But when the adjective comes before a noun, it does take an ending:

• die freundlichen Leute
• eine ruhige Atmosphäre

So:

• after sein: freundlich, ruhig
• before a noun: freundliche / freundlichen, ruhige, etc.

Because the verb has to agree with the subject.

• die Leute is plural → sind
• die Atmosphäre is singular → ist

So:

• Die Leute sind ...
• Die Atmosphäre ist ...

This is the same basic idea as people are vs the atmosphere is in English.

Here ruhig means something like calm, peaceful, or quiet.

With Atmosphäre, it usually describes the overall feeling of a place, not just noise level. So it suggests that the area feels relaxed and not hectic.

Depending on context, ruhig can mean:

• quiet
• calm
• peaceful
• not busy
• not stressful

So in this sentence, it is more about the mood of the neighborhood than about literal silence.

They are:

• der Stadtteil → masculine
• die Atmosphäre → feminine

A useful tip: Stadtteil is a compound noun, and the gender usually comes from the last part of the compound.

• der Teil = part
• so der Stadtteil = district / part of a city

For Atmosphäre, you simply have to learn it as die Atmosphäre.

Usually, no. In standard German, when und joins two main clauses, you normally do not need a comma.

So the more typical version would be:

In diesem Stadtteil sind die Leute freundlich und die Atmosphäre ist ruhig.

You may sometimes see a comma for style or clarity, but as a learner, it is safest to remember:

• before und connecting main clauses: usually no comma

Yes. German is flexible, as long as you keep the verb-second rule in main clauses.

For example, you can also say:

Die Leute sind in diesem Stadtteil freundlich und die Atmosphäre ist ruhig.

That is also correct. The difference is mostly emphasis:

• In diesem Stadtteil ... emphasizes the place
• Die Leute ... emphasizes the people

So the original sentence is not the only possible word order, but it is very natural German.

Often yes, but not always exactly.

Stadtteil literally means part of a city. Depending on context, it can be translated as:

• district
• neighborhood
• area
• part of town

A Stadtteil can sometimes sound a bit more official or administrative than English neighborhood, but in many everyday contexts, neighborhood is a very good translation.