De verpleegkundige controleerde het bloed omdat de klachten aanhielden.

Because verpleegkundige is a de-word when it means nurse.

A few useful points:

• de verpleegkundige = the nurse
• It can refer to a male or female nurse
• Dutch job titles are often de-words

Also, verpleegkundige can be both:

• a noun: nurse
• an adjective in some contexts: nursing or related to nursing

But in this sentence, it is clearly a noun.

Controleerde is the simple past of controleren.

So:

• controleren = to check / to examine
• ik controleer = I check
• hij/zij controleerde = he/she checked

This is a weak verb, so in the past tense it takes -de or -te. Here it takes -de, giving controleerde.

Why -de and not -te? Because the verb stem ends in a sound that does not belong to the 't kofschip group, so Dutch uses -de.

Because bloed is a het-word.

So:

• het bloed = the blood

In Dutch, singular nouns usually take either de or het, and you often just have to learn which one goes with each noun.

Here:

• het bloed is the direct object of controleerde

Because omdat introduces a subordinate clause.

In Dutch subordinate clauses, the conjugated verb usually goes to the end.

So the sentence is built like this:

• Main clause: De verpleegkundige controleerde het bloed
• Subordinate clause: omdat de klachten aanhielden

Compare:

• Main clause word order: subject + verb + rest
• Subordinate clause after omdat: omdat
  • subject + rest + verb

That is why you get:

• omdat de klachten aanhielden

and not:

• omdat de klachten aanhielden is correct
• omdat de klachten hielden aan is not the normal order here

Because the subject is de klachten, which is plural.

So the verb must also be plural in the past tense:

• de klacht hield aan = the complaint persisted
• de klachten hielden aan = the complaints persisted

Here, Dutch uses the plural past form -en:

• aanhielden

Here, aanhouden means to continue, to persist, or to last.

So:

• de klachten hielden aan = the symptoms/complaints continued
• de klachten aanhielden = the symptoms/complaints persisted

This is a separable verb in many contexts:

• infinitive: aanhouden
• main clause: De klachten hielden aan
• subordinate clause: omdat de klachten aanhielden

That shift is very common with separable verbs in Dutch:

• In a main clause, the prefix often separates.
• In a subordinate clause, the verb usually appears as one whole form at the end.

Because klacht is a de-word, and the plural of klacht is klachten.

So:

• de klacht = the complaint / symptom
• de klachten = the complaints / symptoms

In Dutch, plural nouns normally take de, even if the singular might be het. In this case, though, the singular is already de klacht.

In medical contexts, klachten often means symptoms rather than complaints in the everyday English sense.

Literally, yes, klachten often translates as complaints. But in medical Dutch it usually means symptoms or health problems.

So in a sentence like this, an English speaker should usually think:

• de klachten hielden aan = the symptoms persisted

not necessarily:

• the complaints continued

That literal translation is possible, but it sounds less natural in English medical context.

Because Dutch often uses the simple past where English might choose different past forms depending on context.

Here, controleerde is just the normal past tense:

• checked

Whether English translates it as checked, was checking, or something else depends on context, but Dutch does not need a separate form here.

So:

• controleerde = simple past
• It does not automatically mean was checking
• It does not automatically mean had checked

Yes. In modern Dutch, verpleegkundige is normally gender-neutral.

So:

• de verpleegkundige can mean the nurse regardless of sex

If the context makes it clear, Dutch may still use pronouns like:

• hij = he
• zij = she

But the noun itself does not have to change.

It could be, but that would be a different tense choice.

Compare:

• De verpleegkundige controleerde het bloed...
  simple past; often used in storytelling or written narration

• De verpleegkundige heeft het bloed gecontroleerd...
  present perfect; very common in spoken Dutch, especially in everyday conversation

Both can be correct, but they do not always sound the same stylistically.

In many written narratives, Dutch prefers the simple past:

• controleerde
• aanhielden

That is why this sentence sounds natural as a past narrative sentence.