Zwelling is een zeldzame bijwerking.

Here zwelling is being used in a general, category-like way: it refers to swelling as a phenomenon or symptom, not to one specific swelling.

So:

• Zwelling is een zeldzame bijwerking. = talking about swelling in general
• De zwelling is een zeldzame bijwerking. = would refer to a specific swelling already known from the context
• Een zwelling is een zeldzame bijwerking. = possible in some contexts, but it sounds more like a swelling as one individual case

In medical or informational language, Dutch often uses this kind of general noun style.

Because bijwerking is a singular countable noun here, so it normally needs an article.

• een bijwerking = a side effect
• de bijwerking = the side effect

So een zeldzame bijwerking means a rare side effect.

Without een, the phrase would be ungrammatical in a normal sentence.

Because it comes before a noun.

In Dutch, adjectives usually take -e when they are placed before a noun. So:

• een zeldzame bijwerking
• de zeldzame bijwerking

But when the adjective comes after a linking verb like zijn, it usually stays in the basic form:

• De bijwerking is zeldzaam.

So:

• attributive adjective: zeldzame
• predicative adjective: zeldzaam

It is a de word:

• de bijwerking
• een bijwerking
• plural: de bijwerkingen

That matters for adjective endings and for choosing the correct article later when you use the definite form.

The sentence follows normal Dutch main clause word order:

• Zwelling = subject
• is = finite verb
• een zeldzame bijwerking = predicate noun phrase

So the structure is:

subject + verb + complement

Dutch main clauses typically put the finite verb in second position. Since Zwelling comes first, is must come next.

No. Dutch nouns are not capitalized the way German nouns are.

Zwelling has a capital letter here only because it is the first word of the sentence.

So in the middle of a sentence, it would be written zwelling, not Zwelling.

Yes. It is related to the verb zwellen, meaning to swell.

The noun zwelling is formed with the common noun ending -ing, which often turns a verb into a noun.

Related forms include:

• zwellen = to swell
• gezwollen = swollen
• zwelling = swelling

This is useful because Dutch often builds word families in a very regular way.

A rough guide:

• een is often pronounced like a weak uhn in normal speech
• zwelling starts with zw, which may feel unusual for English speakers; the w is Dutch, not English w
• bijwerking begins with bij, which sounds roughly like bye

A more careful pronunciation guide:

• zwelling ≈ ZVEL-ling
• is ≈ iss
• een ≈ uhn
• zeldzame ≈ ZELT-saa-muh
• bijwerking ≈ BYE-ver-king

A key point: Dutch w sounds softer than English w, closer to somewhere between English w and v.

Yes, but the focus changes.

• Zwelling is een zeldzame bijwerking.
  This identifies swelling as one rare side effect.

• De bijwerking is zeldzaam.
  This means the side effect is rare, referring to a specific side effect already understood from context.

You could also say:

• Zwelling is zeldzaam.

But that means swelling is rare in general, and it leaves out the idea of side effect.

So the original sentence is more precise in a medical context.

The plural forms are:

• zwellingen = swellings
• bijwerkingen = side effects

So a plural version would be:

Zwellingen zijn zeldzame bijwerkingen.

Changes to notice:

• zwelling → zwellingen
• is → zijn
• een disappears, because plural indefinite nouns usually have no article
• bijwerking → bijwerkingen
• zeldzame stays zeldzame

Both are possible.

• In careful speech, een can be pronounced more clearly.
• In everyday speech, it is very often reduced to something like uhn or even a very weak vowel sound.

This is normal and not sloppy. Dutch unstressed een is often much weaker than English speakers expect.

Also note the difference between:

• een = a/an
• één = one

The spelling with an accent, één, is used when you want to stress the number one specifically.