Learn Hardcore German: Meine Freundin passt auf die Kinder auf; sie dürfen keinesfalls an den Herd. - My friend female watches the children; they must by no means go to the stove.

Because aufpassen is a separable verb and, in the meaning “to look after,” it takes the preposition auf + accusative. In a main clause, the separable prefix auf goes to the end, while the prepositional object auf die Kinder sits before it. So you get:

Main clause: Meine Freundin passt auf die Kinder auf.
Subordinate clause: …, weil meine Freundin auf die Kinder aufpasst.

The “double auf” looks odd to English speakers but is perfectly normal German.

Which case does auf take in auf die Kinder (aufpassen)?

Accusative. Treat aufpassen (auf + Akk.) as a fixed pattern. Examples with pronouns:

Correct: auf mich/ihn/sie/uns/euch/sie aufpassen
Not correct here: auf mir/ihm/ihnen aufpassen

In sie dürfen, does sie mean “she,” “they,” or formal “you”?

“they.” The verb dürfen is plural; if it referred to “she,” it would be sie darf. It isn’t formal Sie either, because it’s lower‑case. You can make it crystal clear by repeating the noun: Die Kinder dürfen keinesfalls …

Why isn’t sie capitalized after the semicolon?

In German, you don’t automatically capitalize the first word after a semicolon. Capitalization follows the normal rules—nouns and names are capitalized; pronouns like sie are not, unless they start a new sentence or are the formal Sie.

Could I use a period or a comma instead of the semicolon?

Yes.

Period: Meine Freundin passt auf die Kinder auf. Sie dürfen keinesfalls an den Herd.
Comma: Meine Freundin passt auf die Kinder auf, sie dürfen keinesfalls an den Herd.

In German, a comma between independent clauses without a conjunction is allowed; a semicolon simply signals a stronger separation than a comma but weaker than a full stop.

What exactly does keinesfalls mean? Could I use nicht or auf keinen Fall instead?

keinesfalls = “under no circumstances / by no means” (fairly formal/emphatic).
auf keinen Fall is a very common, more conversational alternative.
nicht simply negates: Sie dürfen nicht … (weaker than keinesfalls).
Don’t say Sie dürfen keinesfalls nicht … in standard German—that’s a double negative.

Also note the difference from niemals (“never,” temporal): niemals says “at no time,” while keinesfalls says “under no circumstances.”

Where can I place keinesfalls in the sentence?

Typical placements:

Neutral: Sie dürfen keinesfalls an den Herd.
Emphatic fronting: Keinesfalls dürfen sie an den Herd. Placing it right before an explicit verb of motion also works: Sie dürfen keinesfalls an den Herd gehen.

Why an den Herd and not am Herd, zum Herd, or auf den Herd?

an den Herd (accusative) = movement toward the stove to be “at” it (danger zone).
am Herd = an dem Herd (dative), a stationary location: “at the stove” (e.g., cooking).
zum Herd (zu + dative) = movement “to(wards) the stove” in a general sense; possible, but in the context of “don’t go near the stove,” an den Herd is idiomatic.
auf den Herd (accusative) = onto the stove’s surface (e.g., Den Topf auf den Herd stellen).

Why is it den Herd (accusative) here?

Because an is a two‑way preposition: it takes

accusative for motion toward a place: an den Herd (gehen/kommen)
dative for location: am Herd (stehen/sein)

Is a verb missing in sie dürfen keinesfalls an den Herd?

It’s an intentional ellipsis. The verb of motion or contact is understood from context. You could make it explicit:

Sie dürfen keinesfalls an den Herd gehen/kommen/fassen. German often omits such obvious verbs with modals.

Does aufpassen mean “look after” or “pay attention”? How do I use it?

Both, depending on structure:

“Pay attention”: Ich passe im Unterricht auf.
“Look after”: aufpassen auf + Akk. → Sie passt auf die Kinder auf. Related verbs:
betreuen (supervise, often professional)
hüten (mind, babysit; more colloquial)
sich kümmern um (take care of, broader care)
achten auf + Akk. (pay attention to; not the usual choice for childcare)

Can I say Meine Freundin passt die Kinder?

No. passen alone means “to fit” or “to be convenient.” You need aufpassen (auf + Akk.): Meine Freundin passt auf die Kinder auf. Examples of passen:

Der Hut passt mir. (The hat fits me.)
Morgen passt mir nicht. (Tomorrow doesn’t suit me.)

Does meine Freundin mean “my girlfriend” or “my (female) friend”?

It’s ambiguous in isolation. In everyday German, singular meine Freundin often implies “girlfriend.” To make “female friend” explicit, say eine Freundin von mir. For a romantic partner you can also say meine Partnerin.

Why is it meine Freundin and not meiner Freundin?

Because it’s the subject (nominative, feminine singular). The possessive inflects accordingly:

Nominative: meine Freundin (subject)
Accusative: meine Freundin (direct object; same form here)
Dative: meiner Freundin (to/for my female friend)

Why are Kinder and Herd capitalized?

All nouns are capitalized in German: die Kinder, der Herd. Adjectives and verbs are not (unless they’re substantivized).

Is Herd the same as Ofen?

der Herd = the cooker/stove (the appliance you cook on; often includes the cooktop and sometimes the oven).
der (Back)ofen = the oven (the baking chamber). In everyday speech, Ofen can also mean a heating stove (wood/coal).