Wymieniamy się tylko wtedy, gdy szew, podszewka i ściągacz są całe.

Tylko wtedy, gdy is a fixed pattern meaning only when.

• gdy by itself means when
• wtedy means then
• together, wtedy, gdy creates a stronger correlation: then, when
• with tylko, the whole phrase becomes only when

So:

• gdy są całe = when they are intact
• tylko wtedy, gdy są całe = only when they are intact

You could sometimes shorten it to tylko gdy, but tylko wtedy, gdy sounds more complete and emphatic.

In this sentence, gdy means when.

• gdy = when
• kiedy = when
• jeśli = if

For many everyday sentences, gdy and kiedy are very close in meaning. Here, gdy sounds slightly more formal or written.

Why not jeśli? Because tylko wtedy, gdy expresses a condition framed as only when. Using jeśli would shift it more toward only if. In English those are often very close, but Polish tends to distinguish them a bit more clearly.

So this sentence is naturally read as:

• We exchange/replace only when... rather than
• We exchange/replace only if...

Literally, wymieniamy się means we exchange with each other or we swap.

Breakdown:

• wymieniać = to exchange, replace
• wymieniamy = we exchange / we are exchanging
• się = reflexive/reciprocal marker

The important point is that wymieniać się usually means a mutual exchange, like:

• Wymieniamy się książkami = We exchange books with each other

So if the intended meaning here is we replace an item or we accept an exchange, many learners will notice that się sounds unexpected.

This is the trickiest part of the sentence.

In standard Polish, if you mean we replace/exchange an item, you would usually expect:

• Wymieniamy tylko wtedy, gdy...

without się.

Why? Because:

• wymieniać coś = to replace/exchange something
• wymieniać się = to exchange with one another

So Wymieniamy się tylko wtedy, gdy... sounds more like We exchange with each other only when..., which probably is not the intended meaning.

If this sentence comes from informal language, customer-service language, or a non-native source, that may explain it. A more standard version would often be:

• Wymieniamy tylko wtedy, gdy szew, podszewka i ściągacz są całe.

They are in the nominative because they are the subject of są całe.

The sentence is saying that these things are intact:

• szew = seam
• podszewka = lining
• ściągacz = cuff / ribbed band / elasticated edge

Since they are the things doing the being intact, Polish uses the nominative.

So grammatically, the structure is:

• szew, podszewka i ściągacz = compound subject
• są = are
• całe = intact / whole

Because the subject contains three nouns joined by i (and):

• szew
• podszewka
• ściągacz

Together they form a plural subject, so Polish uses the plural verb:

• są = are

If there were only one noun, you would use jest:

• Szew jest cały = The seam is intact

But with three items:

• Szew, podszewka i ściągacz są całe = The seam, lining, and cuff are intact

Because Polish adjectives must agree with the whole subject, and here the subject is a plural group of things.

The singular forms would be:

• cały for a masculine singular noun
• cała for a feminine singular noun
• całe for a neuter singular noun

But here we have a mixed group of non-persons, so Polish uses the non-masculine-personal plural form:

• całe

That is why we get:

• szew, podszewka i ściągacz są całe

This is not cali, because cali is the masculine-personal plural form, used for groups of men or mixed groups including men:

• Mężczyźni są cali = The men are unharmed / intact

For objects and things, Polish uses całe.

Yes, literally całe means whole, but in this sentence it is better understood as:

• intact
• undamaged
• not torn
• still in one piece

That is very common in Polish. With clothing or physical objects, cały / cała / całe often means that something has not been damaged.

So here są całe is more natural in English as:

• are intact rather than the more literal
• are whole

Here ściągacz is a clothing term. It usually refers to a:

• ribbed cuff
• ribbed waistband
• stretchy knitted band at the edge of a garment

Because it appears with szew (seam) and podszewka (lining), the clothing meaning is clearly intended.

Be careful: ściągacz can mean other things in other contexts, for example:

• a cheat sheet
• something used for pulling/tightening

But in this sentence it is definitely part of a garment.

Polish often uses the present tense for general rules, policies, and repeated actions.

So wymieniamy does not necessarily mean we are exchanging right now. It can also mean:

• we exchange
• we replace
• we do exchanges
• our policy is to exchange

This is similar to English:

• We only exchange items when...

It is a general statement, not a description of one single action happening at this moment.

Because this sentence describes a general rule or repeated practice, Polish normally uses the imperfective verb.

• wymieniać = imperfective
• wymieniamy = we exchange / we replace (habitually, generally)

A perfective verb such as wymienić would not work the same way in the present tense, because Polish perfective present usually has a future meaning.

So:

• wymieniamy = general present policy
• wymienimy = we will replace / exchange

That is why wymieniamy is the natural choice for a policy statement.

No, Polish word order is fairly flexible, but the chosen order sounds natural and clear.

This version:

• Wymieniamy się tylko wtedy, gdy szew, podszewka i ściągacz są całe.

puts the main statement first and then adds the condition.

You could rearrange parts for emphasis, for example:

• Tylko wtedy wymieniamy..., gdy...
• Gdy szew, podszewka i ściągacz są całe, wymieniamy...

But the original order is normal for stating a rule.

The one thing that matters more than word order here is the earlier point about się: if the intended meaning is we replace/exchange an item, many speakers would prefer wymieniamy without się.