Jeśli rękaw jest mokry, nie wkładaj sukienki do szafy, tylko zostaw ją na krześle.

Jeśli means if. It is basically the same as jeżeli.

• jeśli is very common and slightly more everyday
• jeżeli can sound a bit more formal or careful

In this sentence, either would work:

• Jeśli rękaw jest mokry...
• Jeżeli rękaw jest mokry...

Both mean the same thing.

Rękaw is the subject of the clause rękaw jest mokry, so it stays in the nominative case, which is the dictionary form.

The structure is simply:

• rękaw = sleeve
• jest = is
• mokry = wet

So Polish is literally saying the sleeve is wet.

Because mokry has to agree with rękaw.

Rękaw is:

• masculine
• singular
• nominative

So the adjective also has to be:

• masculine
• singular
• nominative

That gives mokry.

Compare:

• mokry rękaw = a wet sleeve
• mokra sukienka = a wet dress
• mokre krzesło = a wet chair

The adjective agrees with the noun it describes, not with some other noun in the sentence.

Wkładaj is the 2nd person singular imperative of wkładać.

So it means:

• wkładaj = put / keep putting
• nie wkładaj = don’t put

It is addressed to one person in an informal way, like English you when speaking to one person.

The verb wkładać is imperfective, and that matters here. In Polish, negative commands are very often made with the imperfective:

• Nie wkładaj... = Don’t put...

That sounds natural and standard.

Because Polish usually prefers the imperfective in negative commands.

• wkładać → nie wkładaj
• włożyć → włóż

A positive command often uses the perfective if you want one completed action:

• Włóż sukienkę do szafy. = Put the dress in the wardrobe.

But for a prohibition, Polish normally goes with the imperfective:

• Nie wkładaj sukienki do szafy. = Don’t put the dress in the wardrobe.

So nie wkładaj is the most natural choice here.

Because after a negated verb, Polish often puts the direct object in the genitive instead of the accusative.

Without negation:

• Wkładaj sukienkę do szafy.

With negation:

• Nie wkładaj sukienki do szafy.

So:

• sukienkę = accusative
• sukienki = genitive

This is a very common pattern in Polish and an important one for learners. In modern everyday Polish, you may sometimes hear accusative after negation in some situations, but genitive is the standard rule to learn here.

Because the preposition do takes the genitive and usually means movement into / to an enclosed place.

So:

• szafa = wardrobe / closet
• do szafy = into the wardrobe / closet

This contrasts with location:

• w szafie = in the wardrobe / closet

So:

• wkładać coś do szafy = to put something into the wardrobe
• coś jest w szafie = something is in the wardrobe

Here tylko means but rather / instead / just in the pattern:

• nie X, tylko Y = not X, but Y

So:

• nie wkładaj sukienki do szafy, tylko zostaw ją na krześle

This is a correction or replacement:

• don’t do X
• do Y instead

Using ale would be less natural in this exact corrective structure. Polish strongly prefers:

• nie..., tylko...

Zostaw is the imperative of the perfective verb zostawić.

That fits well because the speaker wants one complete action:

• leave it on the chair

Compare:

• zostaw = leave it
• zostawiaj = keep leaving / be leaving / leave repeatedly

In positive commands, Polish often uses the perfective when the goal is a single finished result. So zostaw ją na krześle sounds natural.

Because ją is the accusative form of the pronoun ona, and here it is the direct object of zostaw.

It refers back to sukienka.

Forms of ona include:

• ona = she
• ją = her / it, as a direct object
• jej = to her / of her, or her as a possessive form depending on context

So here:

• zostaw ją = leave it

Not:

• zostaw jej

Also notice something important: even though the noun appeared earlier as sukienki because of negation, the pronoun changes according to its role in its own clause. After zostaw, the direct object is accusative, so ją is correct.

Krześle is the locative singular of krzesło.

After na, Polish often uses:

• locative for location: on
• other case patterns in different meanings, but here the idea is location

So:

• na krześle = on the chair

This fits the idea that the dress is being left in that location.

The noun changes like this:

• krzesło = chair
• na krześle = on the chair

The vowel/consonant pattern changes a little, which is normal in Polish declension.

These are commands to one person, in the informal singular.

So the speaker is talking to:

• one person
• not in a formal Pan/Pani way

If you wanted plural informal, you would say:

• nie wkładajcie
• zostawcie

If you wanted a polite/formal version, Polish would usually use something like:

• proszę nie wkładać sukienki do szafy, tylko zostawić ją na krześle or
• niech pan / niech pani nie wkłada sukienki do szafy, tylko zostawi ją na krześle

There are two commas for two different reasons.

1. After Jeśli rękaw jest mokry

   • This is a subordinate if-clause
   • In Polish, such clauses are separated by a comma from the main clause

2. Before tylko

   • This comma marks the contrast in the structure nie..., tylko...
   • It helps show: not this, but that instead

So the punctuation is completely standard:

• Jeśli rękaw jest mokry, nie wkładaj sukienki do szafy, tylko zostaw ją na krześle.

Grammatically, rękaw just means sleeve.

In context, we understand it as the sleeve of the dress because the sentence then talks about sukienka. Polish often leaves that relationship unstated when it is obvious.

A more explicit version could say something like:

• Jeśli rękaw sukienki jest mokry...

But that is not necessary here, because the meaning is already clear from context.