Learn Hardcore German: Nachts träume ich oft vom Meer, denn das beruhigt mich. - At night I often dream of the sea, because it calms me.

Nachts means “at night” (habitually). It’s an adverb formed from die Nacht with an -s, similar to morgens (in the mornings), mittags, abends.
It’s normally lowercase (nachts). It’s capitalized here only because it starts the sentence.
Nachts suggests a general habit; in der Nacht can refer to a specific night or a specific occurrence. Both are correct, but the nuance differs slightly.
Very formal/poetic: des Nachts (rare in everyday speech).

Why is it träume ich and not ich träume?

German main clauses are Verb-Second (V2). When you put a time adverbial like Nachts first, it takes the first position, so the finite verb träume must come next (second), and the subject ich follows: Nachts | träume | ich …. Without fronting, you can say Ich träume nachts ….

Where can I place oft in this sentence?

Common and natural options include:

Nachts träume ich oft vom Meer.
Ich träume nachts oft vom Meer.
Oft träume ich nachts vom Meer. Less natural: placing oft after the prepositional phrase (e.g., … vom Meer oft) or far from the verb. Frequency adverbs usually sit near the verb and before long complements.

What is vom here?

Vom is the contraction of von dem. The preposition von governs the dative case, and das Meer (neuter) in the dative is dem Meer. So von dem Meer → vom Meer. You can write von dem Meer, but the contraction vom is idiomatic and preferred.

Why is it träumen von and not träumen über?

The default for “to dream of/about something (in sleep or as a wish)” is von: von etwas träumen.
Über can appear when the dream is “about a topic” (e.g., Ich habe über die Prüfung geträumt), but many speakers still prefer von. Stick with von as your safe choice.
Pattern to express a wish: davon träumen, etwas zu tun (e.g., Ich träume davon, ans Meer zu fahren).

Why is dem Meer in the dative?

Because von always takes the dative. The noun das Meer becomes dem Meer in dative singular; hence the contraction vom Meer.

What’s the difference between das Meer and der/die See?

das Meer = the sea/ocean (standard word).
der See = the lake.
die See = the sea (poetic/nautical; less common in everyday use). In this sentence, vom Meer clearly means “of the sea/ocean,” not a lake.

How does denn differ from weil?

Both can mean “because.”
Denn is a coordinating conjunction: it links two main clauses and keeps normal V2 word order in the second clause. Example: …, denn das beruhigt mich.
Weil is a subordinating conjunction: it sends the verb to the end. Example: …, weil das mich beruhigt.
Style: denn feels a bit more written/formal and factual; weil is more common in speech and focuses on cause.

Is the comma before denn mandatory?

Yes. In German, a comma is obligatory before denn, aber, sondern, doch when they connect two clauses. With und or oder, the comma can be optional; with denn, it’s required.

What does das refer to in denn das beruhigt mich? Could I use es?

Here, das is a demonstrative pronoun referring to the entire preceding idea/situation (dreaming of the sea at night). It’s like English “that.”
Es is also possible: …, denn es beruhigt mich. Using das adds a bit more pointing/emphasis (“that, specifically”); es is more neutral.

What’s the difference between das and dass here?

Das (one s) is an article or a demonstrative/relative pronoun. In the sentence, it’s a demonstrative pronoun (“that”).
Dass (double s) is a conjunction meaning “that” introducing a subordinate clause (e.g., Ich weiß, dass das Meer mich beruhigt). A quick check: if you can replace it with dieses (“this/that one”), use das; if it introduces a clause with the verb at the end, it’s dass.

Why is it beruhigt and not another form?

Beruhigt is the 3rd person singular present of beruhigen (to calm). The subject of the clause is das (singular), so the verb agrees: das beruhigt. Present tense is used here to express a general/habitual truth.

Why mich and not mir?

Mich is accusative; mir is dative.
Beruhigen is a transitive verb that takes a direct object in the accusative: jemanden beruhigen → das beruhigt mich.
Compare: verbs that take dative (no direct object) include helfen (jemandem helfen) or impersonal expressions like Es ist mir kalt.

Can I rephrase the whole sentence in other natural ways?

Yes, for example:

Ich träume nachts oft vom Meer, denn das beruhigt mich.
Oft träume ich nachts vom Meer, weil das mich beruhigt.
Nachts träume ich oft vom Meer. Das beruhigt mich.
Nachts träume ich häufig vom Meer, denn es beruhigt mich. All are idiomatic, with slight shifts in emphasis or register.