Learn Hardcore German: Ein Problem kann plötzlich wieder auftreten.

Because Ein Problem is the grammatical subject here and you’re speaking about any problem in general, not a specific one. In German, the subject takes the nominative case. Das Problem would be definite (“the problem”), while ein Problem is indefinite (“a problem”).

Why is kann in second position, and auftreten at the very end of the sentence?

This follows the rule for main clauses with a modal verb: the finite verb (kann) goes in the second position, and the main verb in its infinitive form (auftreten) is pushed to the end.

What kind of verb is auftreten, and why isn’t the prefix auf- split off and placed at the end?

auftreten is a separable-prefix verb (auf·treten). When used with a modal (here kann), you keep the full infinitive together at the end. Without a modal, you would separate it:
• Ein Problem tritt plötzlich wieder auf.

What parts of speech are plötzlich and wieder, and why are they placed between kann and auftreten?

Both plötzlich (suddenly) and wieder (again) are adverbs modifying auftreten. In German, adverbs normally follow the finite verb, so after kann you can list one or more adverbs before the final infinitive.

Can I swap the order of plötzlich and wieder? Would Ein Problem kann wieder plötzlich auftreten be correct?

Yes, it’s grammatically acceptable, but the nuance shifts. plötzlich wieder emphasizes suddenness first, then repetition. wieder plötzlich highlights the repetition before the suddenness. Native speakers tend to prefer plötzlich wieder in this context.

What’s the difference between wieder and erneut, and can I use erneut here?

Both mean “again,” but wieder is more common in everyday speech, while erneut is a bit more formal or literary. You could say Ein Problem kann erneut plötzlich auftreten, but it sounds more formal than using wieder.

What does auftreten exactly mean in this sentence? Does it always translate as “to occur”?

Here auftreten means “to occur” or “to arise” (e.g. a problem or symptom appears). It doesn’t mean “to step up” or “to walk up” in this context. You’ll often see it in technical or medical contexts, like Ein Fehler kann auftreten (“An error can occur”).

Why is Problem capitalized in German?

In German, every noun is always capitalized, regardless of where it appears in a sentence. This rule helps you spot nouns immediately.