De radiator heeft een radiatorknop.

In Dutch, radiator is a de-word, so the correct definite article is de.

Dutch nouns have two common-gender article patterns:

• de for common gender
• het for neuter gender

Unfortunately, for many nouns, you simply have to learn whether they take de or het.
So:

• de radiator = correct
• het radiator = incorrect

Heeft is the correct present-tense form of hebben for de radiator, which is third-person singular.

The verb hebben is conjugated like this in the present tense:

• ik heb
• jij/u hebt or heeft (depending on structure/formality)
• hij/zij/het heeft
• wij hebben
• jullie hebben
• zij hebben

Since de radiator means the radiator, it behaves like hij/zij/het in grammar, so you use heeft.

Een is the indefinite article, meaning a or an.

So:

• de radiator = the radiator (specific radiator)
• een radiatorknop = a radiator knob (not a specific one already identified)

This is very similar to English:

• The radiator has a knob.

If the sentence used de radiatorknop, it would mean the radiator has the radiator knob, which would usually sound strange unless a very specific knob had already been mentioned.

Dutch very often forms compound nouns by writing them as one word.

So:

• radiator
  • knop → radiatorknop

This is one of the most important differences from English, where we often write similar combinations as two words:

• radiator knob in English
• radiatorknop in Dutch

Other common Dutch examples:

• huisdeur = house door / front door
• keukentafel = kitchen table
• treinstation = train station

If you wrote radiator knop as two separate words in Dutch, that would usually be considered incorrect.

In Dutch compounds, the last part is usually the main noun, and the earlier part describes it.

So in radiatorknop:

• knop is the main noun
• radiator- tells you what kind of knob it is

So it means:

• a knob for a radiator
• a radiator knob

This is the same logic as:

• keukendeur = kitchen door
• autosleutel = car key

The last element is usually what the thing is.

This sentence follows the basic Dutch main-clause word order:

Subject + verb + object

So:

• De radiator = subject
• heeft = verb
• een radiatorknop = object

This is the same basic pattern as English:

• The radiator has a radiator knob.

Dutch main clauses typically put the finite verb in the second position, and in a simple sentence like this that gives you:

• De radiator heeft een radiatorknop.

Yes, grammatically you could, but it changes the meaning.

• De radiator heeft een radiatorknop. = The radiator has a radiator knob.
• De radiator heeft radiatorknoppen. = The radiator has radiator knobs / multiple knobs.

The original sentence uses een and the singular noun radiatorknop, so it refers to one knob.

Plural:

• singular: de radiatorknop
• plural: de radiatorknoppen

Notice that the plural gets -en here.

In Dutch, common nouns are not capitalized, just like in English.

So:

• de radiator
• een radiatorknop

You only use capital letters for things like:

• the first word of a sentence
• proper names
• some abbreviations

So Radiatorknop with a capital letter would normally be wrong unless it appeared at the beginning of a sentence.

Heeft is usually pronounced roughly like hayft, but with a Dutch vowel sound.

A helpful approximation for English speakers is:

• heeft ≈ hayft

A few pronunciation notes:

• ee in Dutch is a long vowel, somewhat like the vowel in day, but usually tenser and without the strong English glide
• f is just f
• t is just t

So try something like:

• heeft → hayft

Not perfect, but close enough for a learner.

It is grammatically correct, but it can sound a little repetitive because both nouns contain radiator:

• De radiator heeft een radiatorknop.

Dutch allows this, and native speakers will understand it easily. But in real-life conversation, people might often choose a simpler wording if the context is obvious, such as:

• De radiator heeft een knop.

That said, if you specifically want to make clear that it is a radiator knob and not some other kind of knob, radiatorknop is perfectly fine.

It takes de: de radiatorknop.

A useful rule for compound nouns in Dutch is that the whole compound usually takes the article of the last noun.

Since:

• de knop

the compound becomes:

• de radiatorknop

So:

• de radiatorknop
• een radiatorknop

Not:

• het radiatorknop