Trojnásobek sazby poplatku je vysoký.

Here is the breakdown:

• trojnásobek — nominative singular
• sazby — genitive singular
• poplatku — genitive singular
• je — 3rd person singular present of být (to be)
• vysoký — nominative singular masculine, agreeing with the subject

So the overall structure is:

• subject: trojnásobek sazby poplatku
• verb: je
• predicate adjective: vysoký

The important thing for learners is that the main head of the subject is trojnásobek, not sazba or poplatek.

Because trojnásobek usually takes the genitive: trojnásobek čeho? = three times / a triple of what?

So:

• dictionary form: sazba
• after trojnásobek: trojnásobek sazby

This is very similar to how some quantity words in Czech are followed by the genitive. English learners often want to keep the next noun in its basic form, but Czech does not do that here.

Because poplatku depends on sazby.

The phrase sazba poplatku means something like the rate of the fee/charge. In Czech, one noun often modifies another noun by using the genitive:

• sazba čeho? — poplatku

So you get a chain:

• trojnásobek sazby poplatku
• a triple of the rate of the fee

This kind of stacked genitive structure is very common in formal or technical Czech.

Because vysoký agrees with trojnásobek, which is masculine singular.

Even though sazba is feminine, it is not the grammatical head of the subject. The head is trojnásobek.

So Czech treats the sentence like this:

• Trojnásobek ... je vysoký.

If the subject were sazba, then you would get:

• Sazba poplatku je vysoká.

That agreement point is very important in Czech.

Trojnásobek is a noun meaning a threefold amount, a triple, or three times the amount of something.

It is not an adjective here. It functions as the subject of the sentence.

Compare:

• trojnásobek sazby — a triple of the rate / three times the rate
• trojnásobná sazba — a triple rate / a threefold rate

So trojnásobek names a quantity or multiple, while trojnásobná describes a noun.

They are related, but they do different jobs:

• trojnásobek — noun

  • trojnásobek sazby
  • a triple / three times the rate

• trojnásobný / trojnásobná / trojnásobné — adjective

  • trojnásobná sazba
  • a triple / threefold rate

• třikrát — adverb

  • třikrát vyšší
  • three times higher

So if you are talking about a multiple as a thing, Czech often uses trojnásobek. If you are describing a noun, use trojnásobný. If you are modifying a verb, adjective, or comparison, use třikrát.

Yes. In Czech, vysoký is used not only for physical height but also for abstract things such as:

• prices
• fees
• taxes
• rates
• numbers
• salaries

So vysoký poplatek, vysoká sazba, and vysoké náklady are all normal.

This works much like English high in high fee, high rate, or high cost.

The given order is the most neutral:

• Trojnásobek sazby poplatku je vysoký.

Czech word order is flexible, but changing it changes emphasis. For example:

• Vysoký je trojnásobek sazby poplatku.
  This sounds marked and puts extra focus on vysoký.

• Je vysoký trojnásobek sazby poplatku.
  Possible in some contexts, but less neutral.

For a learner, the original version is the safest standard order.

Yes, it is grammatical and understandable, but it sounds fairly formal or technical. It fits best in contexts like:

• administration
• finance
• legal language
• official reports

A native speaker might use something simpler in everyday speech, depending on the exact meaning they want. But the sentence itself is perfectly normal in a formal register.

The slightly heavy feeling comes mainly from the stacked noun phrase trojnásobek sazby poplatku, which is typical of official style.