Когда шов снова стал ровным, я поняла, что старая куртка ещё пригодится.

After verbs like стать / становиться (to become), Russian often uses the instrumental case for the word that describes the new state.

So:

• шов стал ровным = the seam became even/smooth

Compare:

• Он стал врачом = He became a doctor
• Она стала спокойной = She became calm

You may sometimes see nominative after стать in informal speech, but instrumental is the standard choice.

The verb here is стать = to become. It is not reflexive.

So:

• шов стал ровным = the seam became even

The ending -ся appears with many Russian verbs, but стать in this meaning does not take it.

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

Here the subject is я, and the speaker is female, so:

• я понял = I understood (male speaker)
• я поняла = I understood (female speaker)

So поняла tells you the speaker is a woman.

Пригодится comes from пригодиться, which means:

• to come in handy
• to be useful
• to prove useful later

So:

• старая куртка ещё пригодится = the old jacket will still come in handy

It often suggests something may be useful in the future, even if it seems old or unnecessary now.

Because the idea is about future usefulness: after fixing the jacket, the speaker realizes it will still be useful.

Пригодиться is a perfective verb, and perfective verbs form the future with one simple form:

• пригодится = will come in handy

There is no present-tense meaning here. A perfective “present” form usually has future meaning.

Here ещё means still.

• старая куртка ещё пригодится = the old jacket will still come in handy

In other contexts, ещё can mean different things, such as:

• more
• another
• again
• still

But in this sentence, still is the natural meaning.

Because it is the subject of the clause:

• старая куртка ещё пригодится

The jacket is the thing that will be useful, so it is in the nominative case.

Что here means that and introduces a subordinate clause.

• я поняла, что... = I understood/realized that...

So the structure is:

• Я поняла = main clause
• что старая куртка ещё пригодится = subordinate clause

This is very common in Russian.

Russian uses commas to separate subordinate clauses.

This sentence has this structure:

• Когда шов снова стал ровным, ... = time clause (When the seam became even again)
• я поняла, ... = main clause
• что старая куртка ещё пригодится = content clause (that the old jacket would still come in handy)

So the commas mark the boundaries between these parts.

Because the sentence starts with a time clause:

• Когда шов снова стал ровным = When the seam became even again

Russian allows flexible word order, and putting the when-clause first is completely natural. It sets the scene before the main action:

• Когда шов снова стал ровным, я поняла...

You could rearrange parts in Russian, but this order is very standard.

Снова means again.

• шов снова стал ровным = the seam became even again

It is very close to опять, and in many contexts they are interchangeable.

A rough tendency:

• снова = neutral/literary again
• опять = often more conversational; sometimes can sound like again, annoyingly

Here снова sounds natural and neutral.

Шов means seam or stitching line, especially in clothing or fabric.

In this sentence, it refers to the repaired part of the jacket:

• Когда шов снова стал ровным... = When the seam became even/smooth again...

It is masculine:

• шов
• ровный шов
• шов стал ровным

Russian has no articles like a or the.

So:

• шов can mean a seam or the seam
• старая куртка can mean an old jacket or the old jacket

The exact meaning comes from context. In this sentence, English naturally uses the seam and the old jacket, but Russian does not need articles to express that.

Yes. In this sentence, realized is often the best English translation.

• я поняла, что старая куртка ещё пригодится
  = I realized that the old jacket would still come in handy

Russian понять covers both to understand and to realize, depending on context.

It depends on context.

Ровный can mean:

• even
• straight
• smooth
• level

For a seam, good translations are usually:

• even
• smooth
• sometimes straight

So шов снова стал ровным suggests the seam looked neat and properly aligned again.

Because both actions happened in the past:

• Когда шов снова стал ровным = When the seam became even again
• я поняла = I realized

The first past event sets the moment when the second past event happened. This is normal in both Russian and English.

The final clause uses the future:

• старая куртка ещё пригодится = the old jacket will still come in handy

because that usefulness is understood as something in the future.