Я уже нашёл номер на обороте карты.

Нашёл is the masculine singular past tense of найти, which means to find.

Russian past tense agrees with the subject in gender and number:

• я нашёл = I found, if the speaker is male
• я нашла = I found, if the speaker is female
• мы нашли = we found

So in this sentence, нашёл tells you the speaker is male.

Уже means already.

In this sentence, Я уже нашёл номер... means I have already found the number...

Its position is fairly natural here, after я, but Russian word order is flexible. You could also hear:

• Я нашёл уже номер...
• Уже я нашёл номер...

But Я уже нашёл... is the most neutral and common way to say it.

Номер is in the accusative case because it is the direct object of нашёл: the speaker found the number.

For many masculine inanimate nouns, the accusative singular looks exactly like the nominative singular. So:

• nominative: номер
• accusative: номер

That is why the form does not change here.

На обороте means on the back side or on the reverse side.

Here, обороте is in the prepositional case because it follows на in a location meaning:

• на обороте = on the back

The base form is оборот. After на when talking about location, it often becomes prepositional:

• оборот → на обороте

So the phrase literally works like on the reverse side.

Карты is in the genitive singular because it depends on обороте.

The full structure is:

• на обороте карты = on the back of the card

This is a very common Russian pattern:

• noun in prepositional after a preposition + noun in genitive to show of
• на столе книги would be different in meaning, but structurally you can compare сторона дома, центр города, оборот карты

So:

• оборот = reverse side
• оборот карты = the reverse side of the card

Yes, it is very natural.

Russian often uses оборот for the reverse side of something like:

• a card
• a page
• a document
• a photo

Other possible phrases exist, such as на обратной стороне карты, which is also very common and slightly more explicit:

• на обратной стороне карты = on the back side of the card

So both are correct:

• на обороте карты
• на обратной стороне карты

The version in your sentence is concise and idiomatic.

By itself, карта can mean either map or card, depending on context.

In this sentence, because it says номер на обороте карты, the natural meaning is card, since numbers are often found on the back of a bank card, ID card, or similar item.

So Russian uses the same word, and context tells you which meaning is intended.

Yes. Russian word order is more flexible than English word order.

The sentence

• Я уже нашёл номер на обороте карты

is the most neutral version.

You could also say:

• Номер на обороте карты я уже нашёл
• Я номер на обороте карты уже нашёл

These all mean roughly the same thing, but the emphasis changes:

• starting with номер emphasizes what was found
• putting уже later can sound more marked or conversational

For learners, the original order is the safest and most natural.

Yes, it is important for pronunciation and sometimes for clarity.

Нашёл is pronounced with yo, not plain e:

• нашёл = na-SHYOL

In many Russian texts, ё is often written as е, so you may see нашел. But it still means нашёл and is pronounced the same way.

For learners, it is helpful to remember the correct form with ё, because it makes pronunciation and recognition easier.