Ein häufiges Symptom ist, dass ich ständig niesen muss.

A common symptom is that I have to sneeze all the time.

Breakdown of Ein häufiges Symptom ist, dass ich ständig niesen muss.

• sein: to be
• ich: I
• dass: that
• müssen: have to
• niesen: to sneeze
• häufig: common
• das Symptom: the symptom
• ständig: all the time

Questions & Answers about Ein häufiges Symptom ist, dass ich ständig niesen muss.

Why is it ein häufiges Symptom and not einen häufigen Symptom?

Because Symptom is neuter in German: das Symptom.
So the phrase is in the nominative neuter singular (it’s the subject of the sentence):

• ein (nominative neuter)
• adjective ending -es → häufiges
• noun: Symptom

Einen häufigen would be accusative masculine, which doesn’t fit here.

How do I know Symptom is neuter, and what’s the plural?

The standard gender is das Symptom (neuter). The plural is die Symptome.
In your sentence, the neuter gender is also signaled by ein häufiges (both point to neuter nominative).

Why is there a comma in ist, dass ...?

German requires a comma before a subordinate clause introduced by dass.
So ist, dass ich ... muss is punctuated that way because the dass-clause begins right after ist.

What exactly is dass doing here?

dass introduces a subordinate clause (a “that-clause”). In this sentence, the dass-clause gives the content of what the symptom is:

• Ein häufiges Symptom ist, dass ich ständig niesen muss.

You can think of it as: “A common symptom is that I have to sneeze constantly.”

Why does muss come at the end: dass ich ständig niesen muss?

In a dass-clause (a subordinate clause), German typically sends the finite verb (the conjugated verb) to the end.
Here the verb is muss (from müssen), so it goes last:

• main clause word order: Ich muss ständig niesen.
• subordinate clause word order: ..., dass ich ständig niesen muss.

Why are there two verbs: niesen and muss?

Because müssen is a modal verb. Modal verbs combine with a second verb in the infinitive:

• niesen = infinitive (“to sneeze”)
• muss = conjugated modal (“must / have to”)

So niesen muss is the typical modal structure (“have to sneeze”).

What does ständig mean here, and where does it go in the sentence?

ständig means “constantly / all the time / repeatedly.”
Its position is flexible, but common and natural placements are:

• dass ich ständig niesen muss (very common)
• dass ich niesen muss, ständig (unusual in this meaning)
• dass ich immer niesen muss (using immer = “always”)

In general, adverbs like ständig often come before the non-conjugated verb part (niesen) or around the “middle field” of the clause.

Why is it ich ... muss and not ich ... müsse?

muss is present indicative: normal statement of fact.
müsse is Konjunktiv I (reported speech style) and would usually appear in contexts like indirect reporting:

• Doctor says: Er müsse ständig niesen. (“He reportedly has to sneeze constantly.”)

Your sentence is a straightforward claim, so muss is correct.

Can I also say Ein häufiges Symptom ist ständiges Niesen?

Yes. That’s a very natural alternative, especially in medical/technical writing.
It turns the action into a noun phrase:

• Ein häufiges Symptom ist ständiges Niesen. (= “A common symptom is constant sneezing.”)

The original sentence with dass is more personal and explicit about “I”.

Could the sentence start with the dass-clause instead?

Yes, you can front the subordinate clause, but it’s a bit heavier stylistically:

• Dass ich ständig niesen muss, ist ein häufiges Symptom.

Notice:

• The comma after the fronted dass-clause is required.
• The finite verb in the main clause (ist) stays in second position.