Learn Hardcore German: Das Wetter beeinflusst die Laune der Kinder, was wir jeden Tag merken. - The weather affects the children’s mood, which we notice every day.

Because was wir jeden Tag merken is a dependent relative clause. In German, every dependent clause is set off by a comma, and its finite verb (here merken) goes to the end.

Why is was used and not das/die/welches?

Was is the relative pronoun used to refer to an entire preceding statement (the whole idea “the weather affects the kids’ mood”), or to neuter pronouns like alles, nichts, etwas. Using die would point to Laune, and das/welches would point to Wetter—both would change the meaning.

Could I say …, wie wir jeden Tag merken instead of …, was …?

Yes. … wie wir jeden Tag merken means “as we notice every day.” It’s a stylistic alternative. Was feels like “which (is something) we notice,” while wie is “as/as we do.”

Why not use dass here?

You can’t put dass in that exact spot, but you can reformulate: Wir merken jeden Tag, dass das Wetter die Laune der Kinder beeinflusst. That uses a content clause (“that …”) instead of a relative clause.

What is the correct word order inside was wir jeden Tag merken?

In subordinate clauses, the finite verb goes last. Place adverbials like jeden Tag before the final verb:

Correct: … was wir jeden Tag merken.
Incorrect: … was wir merken jeden Tag.

Which cases are used in Das Wetter beeinflusst die Laune der Kinder?

Das Wetter: nominative (subject, neuter).
die Laune: accusative (direct object, feminine; nominative and accusative are both die).
der Kinder: genitive plural (“of the children”).

Can I say die Laune von den Kindern instead of die Laune der Kinder?

It’s understandable, but less elegant and often discouraged in formal German. Prefer the genitive: die Laune der Kinder. If you must use von, die Laune von Kindern (without den) sounds more general.

Why singular die Laune and not plural die Launen?

Singular die Laune der Kinder expresses the children’s overall mood as a collective. Die Launen der Kinder emphasizes distinct, individual moods or mood swings. Both can be correct; the singular is more idiomatic here.

What’s the difference between Laune and Stimmung?

Laune: a person’s short-term mood (good/bad mood).
Stimmung: broader mood or atmosphere—of a person, group, or place. You could say die Stimmung der Kinder, but Laune fits the day-to-day ups and downs better.

Is beeinflusst the best verb here? Any alternatives?

Beeinflussen is perfect and takes the accusative. Alternatives:

wirkt sich auf … aus (+ Akk): “has an effect on”
wirkt auf … (+ Akk): “affects”
bestimmt/prägt …: “determines/shapes” Avoid beeinträchtigt unless you want the explicitly negative “impairs.”

What’s the difference between merken, bemerken, and sich merken?

merken: to notice/perceive. Wir merken das jeden Tag.
bemerken: to notice/observe (often a bit more formal or explicit). Wir bemerken das jeden Tag.
sich merken: to memorize. Wir merken uns das. (different meaning)

Why is jeden Tag accusative?

Time expressions of duration/frequency commonly use the accusative without a preposition: jeden Tag, letzten Winter, nächste Woche. You could also say täglich.

Can German drop the pronoun wir in was wir jeden Tag merken?

No. Standard German is not a pro-drop language. The subject pronoun wir must be stated.

How do you pronounce beeinflusst and Laune?

beeinflusst: be-ein-flusst, approx. [bə-ʔaɪ̯n-flʊst]. The ei sounds like “eye,” the u like the vowel in “put,” and there’s a little glottal break before ein.
Laune: Lau-ne, approx. [ˈlaʊ̯-nə]. The au sounds like “ow” in “now.”

Why is it Das Wetter with the article when speaking generally?

German typically uses the definite article with general, abstract, or generic singular nouns: das Wetter, die Musik, der Mensch. Omitting the article here would be unidiomatic.