Účtovali chybnou částku.

Czech often omits subject pronouns when they are not needed, because the verb form already tells you who the subject is.

In Účtovali chybnou částku, the verb účtovali already means they charged / they billed, so adding oni (they) would usually be unnecessary.

• Účtovali chybnou částku. = They charged an incorrect amount.
• Oni účtovali chybnou částku. = also possible, but oni adds emphasis, like they were the ones who charged the wrong amount.

Here, účtovali means something like:

• they charged
• they billed
• they invoiced

The basic verb is účtovat.

Depending on context, účtovat can mean:

• to charge a customer
• to bill
• to invoice
• sometimes more broadly to account for / keep accounts, but that is not the meaning here

So in this sentence, it refers to charging someone a certain sum of money.

The ending -li shows that this is past tense plural, and specifically that the implied subject is masculine animate plural or a mixed group including at least one male person.

So:

• účtovali = they charged (masculine animate / mixed group)
• účtovaly = they charged (all-feminine or non-masculine-animate plural)

Examples:

• Prodavači účtovali chybnou částku. = The salesmen/shop assistants charged an incorrect amount.
• Pokladny účtovaly chybnou částku. = The cash registers charged an incorrect amount.
  Here pokladny is not masculine animate, so it would be účtovaly.

In your sentence, since the subject is omitted, účtovali suggests that the hidden they refers to people.

Because chybnou částku is the direct object of the verb účtovali, so it has to be in the accusative case, not the nominative.

Dictionary/basic form:

• chybná částka = an incorrect amount
  This is nominative singular.

In the sentence:

• účtovali chybnou částku = they charged an incorrect amount
  Here it becomes accusative singular:
  • chybná → chybnou
  • částka → částku

So the sentence uses the form required by the verb.

Částku is accusative singular.

The verb účtovat takes a direct object: you charge or bill something. That something goes into the accusative.

So:

• částka = nominative singular
• částku = accusative singular

Examples:

• Ta částka je chybná. = That amount is incorrect.
  Here částka is the subject, so nominative.
• Účtovali chybnou částku. = They charged an incorrect amount.
  Here částku is the object, so accusative.

Yes. Účtovat is the imperfective verb.

Its perfective partner is usually naúčtovat.

Very roughly:

• účtovali = they were charging / they charged
  (focus more on the process, habit, or just a neutral past statement depending on context)
• naúčtovali = they charged up / they billed
  (focus more clearly on a completed result)

In real usage, both can often translate into English as they charged.

Compare:

• Účtovali chybnou částku. = They were charging / charged an incorrect amount.
• Naúčtovali chybnou částku. = They ended up charging an incorrect amount / They billed the wrong amount.

If the important point is the completed mistaken charge, Czech often prefers naúčtovali. But účtovali is still natural in many contexts.

Yes. Czech word order is more flexible than English, because case endings show grammatical roles.

The neutral order here is:

• Účtovali chybnou částku.

But you could also see:

• Chybnou částku účtovali.
• Částku účtovali chybnou.
  This last one is much less neutral and depends strongly on context.

Changing the order usually changes emphasis, not the basic meaning.

For example:

• Účtovali chybnou částku. = neutral statement
• Chybnou částku účtovali. = emphasis on the incorrect amount

So yes, the sentence can be reordered, but the original version is the most straightforward.

A helpful approximate pronunciation is:

OOCH-toh-vah-lee KHI-bnoh CHAHST-koo

A few notes:

• Ú is a long u sound.
• č sounds like ch in church.
• ch in chybnou is the Czech sound like the ch in German Bach or Scottish loch.
• á is a long a.
• Czech stress is usually on the first syllable of each word:
  • Účtovali
  • chybnou
  • částku

The cluster účt- may feel difficult at first. Learners often find it easier to say it slowly first:

• úč-to-va-li

The basic form is chybný.

That adjective means:

• incorrect
• mistaken
• faulty

Like other Czech adjectives, it changes according to gender, number, and case.

Here is the singular nominative:

• masculine: chybný
• feminine: chybná
• neuter: chybné

Because částka is a feminine singular noun, and here it is in the accusative, the adjective becomes:

• chybnou částku

The basic form is částka, which means amount or sum.

It is a feminine noun.

In this sentence:

• částka → nominative singular
• částku → accusative singular

Useful forms:

• ta částka = the amount
• bez částky = without the amount (genitive)
• o částce = about the amount (locative)

For this sentence, the important point is that částku is the object form.

Yes, potentially.

Czech past tense with an imperfective verb can sometimes correspond to different English translations depending on context, including:

• they charged
• they were charging
• they used to charge

So Účtovali chybnou částku by itself does not always force just one English tense choice. The wider context decides.

For example:

• In a one-time event, it may mean They charged an incorrect amount.
• In a description of an ongoing situation, it may mean They were charging an incorrect amount.
• In a habitual context, it may mean They used to charge an incorrect amount.

That is one reason why Czech aspect is important, but context still matters a lot.