Heute gehe ich mit derselben Nachbarin zur Post.

Because in a normal German main clause, the finite verb must be in second position.

So when Heute is placed first for emphasis or topic, the verb gehe still has to stay second, and the subject ich comes after it:

• Heute gehe ich ...

This is often called inversion in English-language teaching, although in German it is just the normal verb-second rule.

Compare:

• Ich gehe heute zur Post.
• Heute gehe ich zur Post.

Both are correct. The difference is mainly what comes first in the sentence.

Heute is a time expression meaning today. It tells you when the action happens.

In this sentence, it is placed first, which gives it a little extra prominence. English can do something similar:

• Today, I’m going ...

But unlike English, German then requires the verb to come immediately after that first element:

• Heute gehe ich ...

It is dative case because the preposition mit always takes the dative.

So:

• mit
  • dative

That affects both words:

• derselben
• Nachbarin

The noun Nachbarin is feminine, and in the dative singular it becomes:

• der Nachbarin = the female neighbor

With derselbe in the dative feminine singular, you get:

• derselben Nachbarin

So the whole phrase means with the same neighbor.

Because dieselbe is not the base form used here after mit.

The word belongs to the family:

• derselbe (masculine)
• dieselbe (feminine)
• dasselbe (neuter)

These forms change depending on gender, number, and case.

Since Nachbarin is feminine and mit requires the dative, the correct form is:

• derselben Nachbarin

A few useful comparisons:

• nominative feminine: dieselbe Nachbarin
• accusative feminine: dieselbe Nachbarin
• dative feminine: derselben Nachbarin

So dieselbe is not wrong in general; it is just the wrong case here.

It means the same.

German derselbe / dieselbe / dasselbe is used when it is literally the identical one, not just a similar one.

So:

• mit derselben Nachbarin = with the same neighbor
  meaning: the very same female neighbor as before or as previously mentioned

This is stronger and more specific than just saying someone similar.

Because Nachbarin is the feminine form of Nachbar.

• der Nachbar = male neighbor / neighbor (masculine)
• die Nachbarin = female neighbor

The ending -in is a very common way to form feminine nouns for people in German.

Examples:

• der Lehrer → die Lehrerin
• der Freund → die Freundin

So Nachbarin tells you the neighbor is female.

In the dative singular feminine, many nouns do not change form. The case is often shown by the article or determiner instead.

So:

• nominative: die Nachbarin
• dative: der Nachbarin

The noun Nachbarin itself stays the same. The change shows up in the word before it:

• der
• derselben

That is very normal in German.

Because zur is the common contraction of:

• zu der → zur

Since Post is feminine:

• die Post

and zu takes the dative, you get:

• zu der Post
• usually contracted: zur Post

This contraction is standard and very common.

Here zur Post usually means to the post office.

German die Post can refer to:

• the postal service / mail
• the post office, depending on context

After gehen, the phrase zur Post gehen is a very common expression meaning to go to the post office.

So in this sentence, learners should understand zur Post as a destination.

There actually is an article there.

Zur already contains the article:

• zur = zu der

So the article der is built into the contraction. That is why you do not see a separate word before Post.

Yes, absolutely.

That version is also correct:

• Ich gehe heute mit derselben Nachbarin zur Post.

The difference is mainly emphasis and information structure:

• Heute gehe ich ... puts today first
• Ich gehe heute ... puts I first

Both are natural German.

In German, all nouns are capitalized.

So:

• Post is capitalized because it is a noun

But:

• heute is normally not capitalized unless it is at the beginning of the sentence
• gehe, ich, mit are not nouns, so they are lowercase

In this sentence:

• Heute is capitalized because it starts the sentence
• Post is capitalized because it is a noun

It is a natural German word order, but not the only possible one.

German often places information in a fairly flexible way, especially after the verb. Here the sentence flows naturally as:

• time first: Heute
• verb: gehe
• subject: ich
• companion: mit derselben Nachbarin
• destination: zur Post

This sounds very normal. Other orders can also be possible depending on emphasis, but this version is straightforward and idiomatic.

The subject is ich.

A quick way to find it is to ask: Who is going?

Answer:

• ich = I

Even though ich comes after the verb here, it is still the subject. That is another good reminder that German word order is not as fixed as English word order.