Цей платіж не мій.

Because платіж is a masculine singular noun, and цей must agree with it.

Ukrainian this changes by gender and number:

• цей = this, masculine
• ця = this, feminine
• це = this, neuter
• ці = these, plural

So:

• цей платіж = this payment
• ця картка = this card
• це повідомлення = this message
• ці документи = these documents

For the same reason: мій agrees with the implied noun платіж, which is masculine singular.

Even though English says mine without showing gender, Ukrainian still reflects the gender/number of the thing being talked about:

• Цей платіж не мій. = This payment is not mine.
• Ця картка не моя. = This card is not mine.
• Це місце не моє. = This place is not mine.
• Ці речі не мої. = These things are not mine.

So мій here really means something like mine (masculine thing).

Yes, from an English point of view it feels like something is missing, but in Ukrainian this is normal.

In the present tense, Ukrainian usually omits the verb to be in sentences like this.

So:

• Цей платіж не мій. literally looks like This payment not mine
• but it means This payment is not mine

If you were speaking about the past or future, a form of to be would appear:

• Цей платіж не був моїм. = This payment was not mine.
• Цей платіж не буде моїм. = This payment will not be mine.

The sentence uses the nominative form.

Breakdown:

• цей — nominative masculine singular
• платіж — nominative singular
• мій — nominative masculine singular

This is common in simple identification/description sentences, especially when the present-tense to be is omitted.

A learner may expect something like an instrumental form after is, because Ukrainian sometimes uses the instrumental after forms of бути. But in short present-tense sentences like this, nominative predicate forms are very common and natural.

Не is the negation particle: it means not.

It goes before the part being negated:

• Цей платіж не мій. = This payment is not mine.

Here it negates мій.

A useful comparison:

• Цей платіж мій. = This payment is mine.
• Цей платіж не мій. = This payment is not mine.

Yes, you can say Це не мій платіж. It is very close in meaning, but the focus is a little different.

• Цей платіж не мій. = This payment is not mine.
  Focus on this specific payment.

• Це не мій платіж. = This is not my payment.
  Focus on what this is / what it belongs to.

Both are natural. The version with цей платіж points more directly to the noun payment as the topic.

Yes. Ukrainian word order is flexible, but changing it usually changes the emphasis, not the basic meaning.

Neutral order:

• Цей платіж не мій.

More emphatic possibilities:

• Не мій цей платіж. = This payment is not mine.
  Stronger emphasis on not mine.

• Мій цей платіж? = Is this payment mine?
  Possible in certain contexts, though less neutral.

For learners, the safest default is:

• Цей платіж не мій.

A rough pronunciation guide:

• Цей ≈ tsey
• платіж ≈ plah-TEEZH
• не ≈ neh
• мій ≈ meey / miy

Put together:

tsey plah-TEEZH neh meey

A few notes:

• Ц sounds like ts
• ж sounds like the s in measure
• і is like ee
• й is like a short y sound

The stress is on the second syllable of платіж: платі́ж.

It is a possessive pronoun, but it behaves a lot like an adjective because it agrees with the noun in gender, number, and case.

That is why you get forms like:

• мій — masculine
• моя — feminine
• моє — neuter
• мої — plural

So in practice, learners often find it helpful to think of it as an adjective-like possessive word, even though grammatically it is a pronoun.

Because the sentence is not saying my payment. It is saying this payment is not mine.

Compare:

• мій платіж = my payment
• цей платіж не мій = this payment is not mine

In the sentence you were given, мій is not directly modifying the noun inside the noun phrase. It is the predicate part of the sentence, telling you something about цей платіж.

That is why it stands separately at the end.

You change both the demonstrative and the possessive to match the noun.

Examples:

• Ця картка не моя. = This card is not mine.
  feminine singular

• Це повідомлення не моє. = This message is not mine.
  neuter singular

• Ці платежі не мої. = These payments are not mine.
  plural

This is a very useful pattern to memorize:

• Цей ... не мій
• Ця ... не моя
• Це ... не моє
• Ці ... не мої

Yes, платіж is a standard word for payment, especially in formal, banking, billing, or transaction-related contexts.

Depending on context, Ukrainian may also use other related words, but платіж is completely natural for things like:

• bank payments
• online payments
• invoice payments
• transaction records

So Цей платіж не мій sounds very natural in situations like checking a statement, a banking app, or a billing issue.