De thermometer bevestigde aanhoudende koorts.

Here bevestigde means confirmed.

It is the simple past singular form of the verb bevestigen.

• bevestigen = to confirm
• ik bevestigde = I confirmed
• de thermometer bevestigde = the thermometer confirmed

A useful thing to know is that bevestigen can also mean to attach / fasten in other contexts. In this sentence, though, it clearly means confirm.

Because thermometer is a de-word in Dutch:

• de thermometer

This is something learners usually just have to memorize with the noun. There is no reliable rule that lets you predict the article every time, so it is best to learn nouns together with de or het.

Because koorts is being used as an uncountable noun here, in a general/indefinite sense.

So Dutch can say:

• aanhoudende koorts = persistent fever

without adding de or een.

If you added an article, the meaning would become more specific:

• de aanhoudende koorts = the persistent fever

That would usually refer to a specific fever already known from the context.

Because when an adjective comes before a noun in Dutch, it usually takes -e.

Here:

• aanhoudend = persistent / ongoing
• before the noun koorts, it becomes aanhoudende

This is especially expected with de-words, and koorts is a de-word:

• de koorts
• aanhoudende koorts

So aanhoudend koorts would be incorrect.

Aanhoudende means persistent, continuing, or ongoing.

It comes from the verb aanhouden, which can have several meanings depending on context. In medical or descriptive language, aanhoudend often means that something continues for a while and does not go away.

So:

• aanhoudende koorts = fever that continues / persistent fever

It follows normal Dutch main clause word order:

• De thermometer = subject
• bevestigde = finite verb
• aanhoudende koorts = direct object

So the pattern is:

• Subject + finite verb + object

That is very similar to standard English word order in a simple statement.

It is usually treated as an uncountable noun in everyday Dutch, much like fever in English.

You normally say:

• koorts hebben = to have a fever

and not something like een koorts in ordinary usage.

Plural forms are possible in very specialized or literary contexts, but for normal learners it is best to think of koorts as an uncountable noun.

Yes. Bevestigen can directly take the thing that is being confirmed.

So:

• iets bevestigen = to confirm something

That makes aanhoudende koorts the direct object of bevestigde.

You could also have a clause after it, for example:

• De thermometer bevestigde dat de koorts aanhield.
• The thermometer confirmed that the fever persisted.

It is understandable, and grammatically it works, but it sounds a bit formal or clinical.

A thermometer does not literally confirm in the way a doctor or a test might, so some speakers might prefer a slightly different wording, such as:

• De thermometer wees op aanhoudende koorts.
  The thermometer indicated persistent fever.

• De meting bevestigde aanhoudende koorts.
  The measurement confirmed persistent fever.

• De thermometer bevestigde dat de koorts aanhield.
  The thermometer confirmed that the fever persisted.

So the original sentence is not wrong, but it is a bit more formal and less everyday-sounding.

Dutch uses the simple past quite naturally in narration, reporting, and written style.

So:

• De thermometer bevestigde aanhoudende koorts.

is a normal way to report what happened.

If you used the perfect tense:

• De thermometer heeft aanhoudende koorts bevestigd.

that would also be grammatical, but it can sound a little more focused on the completed result or on its relevance to the present moment.

For many learners, the important point is that Dutch often uses the simple past in places where English might also consider using the present perfect.