Мне нужен не обмен, а полная отмена заказа, потому что эти джинсы слишком тесные, чтобы носить их каждый день.

Russian often expresses need with:

[person in the dative] + нужен / нужна / нужно / нужны + thing needed

So Мне нужен... literally feels like To me, ... is needed.

That is why Russian uses мне instead of a nominative subject like я.

Мне is the dative case of я.

In this structure, the person who has the need is put in the dative:

• Мне нужен обмен = I need an exchange
• Ей нужна помощь = She needs help
• Нам нужны билеты = We need tickets

So я нужен would mean something completely different: I am needed.

This is because нужен appears before the contrast не обмен, а... and agrees with обмен, which is masculine.

• обмен → masculine → нужен
• отмена → feminine → нужна

So the sentence starts with agreement to обмен: Мне нужен не обмен, а полная отмена заказа...

A very natural way to avoid this gender mismatch is to rephrase: Мне нужна полная отмена заказа, а не обмен.

That version makes the agreement simpler for learners.

Не X, а Y means not X, but Y.

It is used to correct or replace one idea with another:

• Не чай, а кофе = Not tea, but coffee
• Не обмен, а полная отмена заказа = Not an exchange, but a full cancellation of the order

Here the speaker is clearly rejecting one solution and stating the one they actually want.

Заказа is genitive singular.

The noun отмена often takes a genitive noun after it to mean cancellation of something:

• отмена рейса = cancellation of the flight
• отмена встречи = cancellation of the meeting
• отмена заказа = cancellation of the order

Compare this with the verb:

• отменить заказ = to cancel the order

With the verb, заказ is accusative. With the noun отмена, it becomes genitive.

Because джинсы is a plural-only noun in Russian, just like jeans in English.

Even one pair is treated as plural:

• эти джинсы = these jeans
• новые джинсы = new jeans

So the demonstrative and adjectives must also be plural:

• эти
• тесные

Here тесные means too tight or too snug.

For clothing, тесный is used when something does not leave enough room on the body.

Useful comparison:

• тесный = tight
• узкий = narrow
• маленький = small

So for jeans that feel uncomfortable because they squeeze, тесные is the right word.

This is the standard way to say too ... to ... in Russian:

слишком + adjective/adverb + чтобы + infinitive/clause

So:

• слишком тесные, чтобы носить их каждый день = too tight to wear every day

It is not meant to be translated word-for-word. Just learn the pattern as a whole.

Because носить means to wear in the sense of wearing something over time or as a habit.

• носить джинсы каждый день = to wear jeans every day

By contrast:

• надеть = to put on
• надевать = to be putting on / to put on repeatedly

So here the meaning is not to put on every day, but to wear as everyday clothing, which is why носить is correct.

Also note:

• одевать usually means to dress someone else

Их means them, referring back to джинсы.

Russian often keeps the object pronoun when it helps make the sentence clear:

• носить их каждый день = wear them every day

You might sometimes omit it in context, but here их makes the reference explicit and sounds natural.

Каждый день is an accusative time expression, meaning every day.

Russian often uses the accusative for repeated time expressions:

• каждый день = every day
• каждую неделю = every week
• каждый год = every year

Here:

• каждый is masculine accusative singular
• it looks the same as nominative because день is masculine inanimate

Because both introduce subordinate parts of the sentence.

• потому что = because
• чтобы here introduces the too...to... construction

So the commas separate the main clause from the subordinate clause(s):

Мне нужен не обмен, а полная отмена заказа, потому что эти джинсы слишком тесные, чтобы носить их каждый день.

This is normal Russian punctuation.

Yes. Russian word order is flexible, and different orders change the emphasis.

The given version emphasizes the customer’s request right away: Мне нужен не обмен, а полная отмена заказа...

A very natural alternative is: Мне нужна полная отмена заказа, а не обмен, потому что эти джинсы слишком тесные, чтобы носить их каждый день.

That version may be easier for learners because нужна clearly agrees with отмена.