Банк проверяет мошенническую операцию.

Because it is the direct object of the verb проверяет (checks / is checking), so it has to be in the accusative case.

• Dictionary form: операция
• Accusative singular: операцию

This is a very common pattern in Russian:

• Банк проверяет операцию. = The bank checks a transaction.
• Я читаю книгу. = I am reading a book.

In both sentences, the thing being acted on goes into the accusative.

Because adjectives in Russian must agree with the noun they describe in:

• gender
• number
• case

Here, операцию is:

• feminine
• singular
• accusative

So the adjective must match it:

• dictionary form: мошеннический = fraudulent
• feminine accusative singular: мошенническую

So:

• мошенническая операция = a fraudulent transaction (nominative)
• проверяет мошенническую операцию = checks a fraudulent transaction (accusative)

Проверяет is a conjugated verb form, while проверить is the infinitive.

• проверять = imperfective to check / to be checking
• проверяет = he/she/it checks or is checking
• проверить = perfective infinitive to check / to verify once, to complete the check

In this sentence, you need a finite verb, so проверяет is correct.

Yes. Проверяет is 3rd person singular present tense of проверять.

It matches банк, which is grammatically singular:

• я проверяю = I check
• ты проверяешь = you check
• он / она / оно проверяет = he / she / it checks
• банк проверяет = the bank checks / is checking

In context, it can mean either:

• The bank checks...
• The bank is checking...

Russian present tense often covers both simple present and present continuous.

Because банк is the subject of the sentence, so it stays in the nominative case.

The basic structure is:

• Банк = subject
• проверяет = verb
• мошенническую операцию = direct object

So банк remains in its dictionary form.

In banking and finance, операция often means a transaction, operation, or financial operation.

So in this sentence, it does not mean a medical operation. In this context, it means something like:

• a bank transaction
• a financial operation
• a payment-related action

This is normal Russian banking vocabulary.

Yes, it is understandable and grammatical. It means fraudulent transaction or fraudulent operation.

In real banking Russian, people might also say things like:

• подозрительная операция = suspicious transaction
• мошенническая транзакция = fraudulent transaction
• банк проверяет операцию на мошенничество = the bank is checking the transaction for fraud

But your sentence is perfectly valid for learning grammar and vocabulary.

Yes. Russian word order is more flexible than English because case endings show the grammatical roles.

The neutral order here is:

• Банк проверяет мошенническую операцию.

But you could also say:

• Мошенническую операцию проверяет банк.

That puts more emphasis on the fraudulent transaction or contrasts the bank with someone else.

Even though the word order changes, the case endings still show that:

• банк is the subject
• мошенническую операцию is the object

Russian has no articles like English a / an / the.

So:

• Банк can mean a bank or the bank
• мошенническую операцию can mean a fraudulent transaction or the fraudulent transaction

The exact meaning depends on context.

English requires articles, but Russian usually leaves that information unstated unless the context makes it clear.

The dictionary forms are:

• банк = bank
• проверять = to check
• мошеннический = fraudulent
• операция = operation / transaction

The sentence uses these actual forms:

• банк — nominative singular
• проверяет — 3rd person singular present
• мошенническую — feminine accusative singular
• операцию — feminine accusative singular

The endings show it.

• мошенническую = feminine singular accusative
• операцию = feminine singular accusative

If it were plural, it would look different. For example:

• Банк проверяет мошеннические операции. = The bank is checking fraudulent transactions.

So the singular endings -ую and -ю tell you it is one transaction.

Проверять is imperfective.

That means it focuses on:

• the process
• repetition
• an action in progress
• a general activity

So Банк проверяет мошенническую операцию suggests:

• the bank is in the process of checking it, or
• the bank checks such transactions

The perfective partner is проверить. That would focus on the action as completed.

For example:

• Банк проверяет операцию. = The bank is checking the transaction.
• Банк проверил операцию. = The bank checked / has checked the transaction.

A helpful pronunciation guide is:

Банк проверя́ет мошенни́ческую опера́цию.

Approximate stress:

• банк
• проверЯет
• мошеннИческую
• оперАцию

A rough English-style approximation would be:

bahnk pra-vye-RYAI-et ma-shen-NEE-chee-skoo-yu a-pye-RA-tsi-yu

The most important thing is to place the stress correctly, especially in:

• проверя́ет
• мошенни́ческую
• опера́цию

No. That would be ungrammatical.

After проверяет, the direct object must be in the accusative case, so both the noun and adjective must change:

• correct: Банк проверяет мошенническую операцию
• incorrect: Банк проверяет мошенническая операция

The second version leaves the adjective and noun in the nominative, which does not fit their role in the sentence.