Het vaste tarief verandert niet.

Because tarief is a het-word in Dutch, so its singular definite article is het.

A few related points:

• singular common-gender nouns take de
• singular neuter nouns take het
• all plural nouns take de

So:

• het tarief
• de tarieven

Unfortunately, whether a noun is a de-word or a het-word often just has to be learned with the noun.

Because Dutch adjectives usually take -e when they come before a noun with a definite article.

Here you have:

• het = definite article
• vast = adjective
• tarief = noun

So the adjective becomes vaste:

• het vaste tarief

This -e ending is very common in Dutch attributive adjectives.

Compare:

• een vast tarief = an indefinite phrase, so no -e here
• het vaste tarief = a definite phrase, so -e appears

Because the subject is het vaste tarief, which is third-person singular, and in the present tense Dutch uses the -t ending for that form.

The infinitive is veranderen.

Present tense:

• ik verander
• jij verandert
• hij/zij/het verandert

So in this sentence:

• subject = het vaste tarief
• verb = verandert

That matches standard present-tense conjugation.

In a simple Dutch main clause, niet often comes near the end, especially when it negates the whole statement.

So:

• Het vaste tarief verandert niet.

This is very natural Dutch word order.

A useful rule of thumb:

• the finite verb usually comes in second position in a main clause
• niet often comes later, frequently after the main information and before any final elements it does not belong to

In this sentence, there is nothing after the verb except the negation, so niet ends up at the end.

Yes, structurally that is the best match.

Dutch usually does not need a separate auxiliary like do/does for negation in the present tense. Instead, it simply uses:

• the conjugated verb
• plus niet

So:

• verandert niet

corresponds to:

• does not change

This is one of the big differences between English and Dutch grammar.

Here vast means something like fixed, set, or not variable.

In Dutch, vast can have several related meanings depending on context, such as:

• fixed
• firm
• secure
• steady
• attached

In vast tarief, it refers to a price/rate that stays the same rather than varying.

So this is a very common business or pricing word.

Yes. That would still be grammatical.

The difference is that vaste adds extra information:

• Het tarief verandert niet = the rate does not change
• Het vaste tarief verandert niet = the fixed rate does not change

With vaste, the sentence may sound slightly more specific, for example contrasting a fixed rate with another kind of rate, such as a variable one.

Yes. Verandert is present tense.

So the sentence states something that is true now, or generally true.

Some useful comparisons:

• Het vaste tarief verandert niet. = present
• Het vaste tarief veranderde niet. = past
• Het vaste tarief is niet veranderd. = present perfect
• Het vaste tarief zal niet veranderen. = future

English speakers often benefit from noticing that the simple present in Dutch is used very naturally for general statements like this.

They are close, but not identical.

• Het vaste tarief verandert niet focuses on an ongoing or general fact: it does not change.
• Het vaste tarief is niet veranderd focuses on the result up to now: it has not changed.

So the first is more like a general statement or policy description, while the second often sounds like you are referring to a period up to the present and confirming that no change has happened.

The plural would be:

• De vaste tarieven veranderen niet.

Notice the changes:

• het tarief → de tarieven
• verandert → veranderen

The adjective stays vaste in the plural.

So plural agreement affects mainly the article and the verb.

A rough pronunciation guide is:

• Het ≈ hut / ut
• vaste ≈ VAHS-tuh
• tarief ≈ tah-REEF
• verandert ≈ vuh-RAHN-durt
• niet ≈ neet

A few pronunciation notes:

• het is often reduced in normal speech
• the stress in tarief is on the second syllable
• r pronunciation varies by region
• niet has a long vowel, somewhat like ee in see, but tenser

A natural rhythm would stress vaste and tarief, with niet also standing out because of the negation.