Learn Hardcore German: Am Donnerstag bringt die Post ein Paket, und ich kaufe frische Brötchen. - On Thursday the post also brings a package, and I buy fresh bread rolls.

In German, when you refer to a specific day, you use an + dative article dem, contracted to am (literally an dem Donnerstag). That gives you am Donnerstag = “on Thursday” (one particular Thursday). If you said donnerstags (with an –s), it would mean “on Thursdays” in general (every Thursday). Just an Donnerstag without the article is not standard for time expressions.

Why does the finite verb bringt come before the subject die Post, and how does this word order work?

German main clauses follow the verb‑second (V2) rule. If you start with an adverbial (Am Donnerstag), that occupies the first position. The finite verb then moves into the second position, pushing the subject to third. So:
1st: Am Donnerstag (time)
2nd: bringt (verb)
3rd: die Post (subject)

Why is die Post treated as a singular feminine noun, and what exactly does it refer to?

Post (the mail service) is a feminine noun in German – you write die Post. It refers to the postal system or mail carrier, not to “post” in English like “blog post.” Because it’s grammatically feminine singular, the verb agrees with that: die Post bringt (she/it brings).

Why is there a comma before und, and do I always need a comma before und when joining two clauses?

In current German orthography, you normally don’t put a comma before und when it simply links two clauses. However, you may insert one if each clause is long or to avoid ambiguity. In Am Donnerstag bringt die Post ein Paket, und ich kaufe frische Brötchen, the comma is optional—you could also write it without a comma and still be correct.

Why are there no articles before frische Brötchen, and why does frisch end in -e here?

When you talk about an indefinite quantity of plural nouns in German, you drop the article altogether. That’s why there’s no die or ein before Brötchen. The adjective then takes a strong ending. For nominative or accusative plural with no article, frisch becomes frische.

How can I tell that Brötchen is plural here, since singular and plural look the same?

German diminutives like Brötchen keep the same form in singular and plural. You know it’s plural because of the adjective ending ‑e (strong inflection) and because of the context ich kaufe frische Brötchen (you buy more than one). If it were singular, you’d say ein frisches Brötchen (“a fresh bun”).

Why do we repeat the subject ich in the second clause (und ich kaufe), instead of just und kaufe frische Brötchen?

In German you almost always need an explicit subject in each main clause. Unlike English, you can’t drop the pronoun before a verb in a coordinate clause. So you must say und ich kaufe, not just und kaufe.

Can I change the word order, for example Die Post bringt am Donnerstag ein Paket, und ich kaufe frische Brötchen? Does it alter the meaning?

Yes. You can place the subject before the time expression: Die Post bringt am Donnerstag ein Paket. German allows flexible word order as long as the verb remains in second position. The meaning stays the same; you only shift emphasis slightly (here the focus moves from when it happens to who is doing it).

If I want to specify that the package is delivered to me, where would I put mir (the dative pronoun)?

You insert the dative pronoun mir before the accusative object. For example:
Am Donnerstag bringt die Post mir ein Paket.
Or you can also say:
Die Post bringt mir am Donnerstag ein Paket.
The usual order in German is Dative before Accusative.