Quando il colletto si bagna, la manica della camicia resta fredda a lungo.

Quando can mean both when and whenever, depending on context.

In this sentence, Quando il colletto si bagna is best understood as When/Whenever the collar gets wet. It introduces a time clause: first the collar gets wet, then the sleeve stays cold for a long time.

So here quando is not asking a question. It is a conjunction meaning when.

No. They are related, but they do not mean the same thing.

• il collo = the neck
• il colletto = the collar of a shirt or other piece of clothing

The ending -etto is often a diminutive ending in Italian. Historically it can suggest something smaller, but in many words it simply forms a standard noun. So colletto is the normal word for collar in clothing.

In si bagna, the verb is bagnarsi, which often means to get wet.

So:

• bagnare = to wet something
  
  • Bagno la camicia = I wet the shirt.
• bagnarsi = to get wet
  
  • La camicia si bagna = The shirt gets wet.

In your sentence:

• il colletto si bagna = the collar gets wet

This is very natural Italian. English often uses get + adjective/past participle where Italian uses a reflexive verb.

Grammatically, yes, it uses the reflexive form, but you should not think of it too literally as the collar wets itself.

In many Italian verbs, si is used in a natural, idiomatic way to express a change of state:

• si rompe = breaks
• si apre = opens
• si chiude = closes
• si bagna = gets wet

So il colletto si bagna is best understood as the collar gets wet, not as a truly self-directed action.

La manica means the sleeve, but della camicia specifies of the shirt.

So:

• la manica = the sleeve
• la manica della camicia = the sleeve of the shirt

Italian often uses this kind of structure very naturally to be specific:

• la porta della casa = the door of the house
• il colore della giacca = the color of the jacket

Could someone say just la manica? Yes, if the context is already clear. But della camicia makes it explicit.

Della is a combination of:

• di = of
• la = the

So:

• di + la = della

In this sentence:

• la manica della camicia = the sleeve of the shirt

This is one of the common Italian contracted forms of di + article:

• del = di + il
• dello = di + lo
• della = di + la
• dei = di + i
• degli = di + gli
• delle = di + le

Resta fredda means stays cold or remains cold.

• è fredda = is cold
  This simply describes a state.
• resta fredda = remains/stays cold
  This emphasizes that the coldness continues over time.

That fits well with a lungo (for a long time).

As for sta fredda, that is generally not the normal choice here. Italian uses restare or rimanere to express to remain/stay in a state:

• resta fredda
• rimane fredda

Both are possible, though resta is what your sentence uses.

Because fredda agrees with la manica, which is feminine singular.

• la manica = feminine singular
• therefore the adjective must be fredda

Compare:

• il colletto è freddo = the collar is cold
• la manica è fredda = the sleeve is cold

Italian adjectives usually agree in gender and number with the noun they describe.

A lungo means for a long time or for long.

So:

• resta fredda a lungo = stays cold for a long time

It is a common expression in Italian. You may also see:

• a lungo termine = in the long term
• pensare a lungo = to think for a long time

Italian uses definite articles much more often than English.

So where English may say:

• When collar gets wet... only in very unusual contexts,
• or more naturally When the collar gets wet...

Italian normally wants the article:

• il colletto
• la manica
• la camicia

This is very standard. In Italian, articles are used regularly with body parts, clothing, and many general nouns where English may sometimes omit them.

The verbs are in the present indicative:

• si bagna
• resta

Italian often uses the present tense for general truths, habits, or repeated situations. This sentence sounds like a general observation:

• When the collar gets wet, the shirt sleeve stays cold for a long time.

So the present tense here is perfectly natural, just like in English general statements:

• When water freezes, it expands.

Yes. In many contexts, restare and rimanere are very close in meaning.

So you could also say:

• Quando il colletto si bagna, la manica della camicia rimane fredda a lungo.

That would still mean essentially the same thing: the sleeve remains cold for a long time.

There can sometimes be small stylistic differences, but for a learner, they are often interchangeable in sentences like this.

The structure is:

• Quando + clause, main clause

More specifically:

• Quando il colletto si bagna = subordinate time clause
• la manica della camicia resta fredda a lungo = main clause

So the pattern is:

When X happens, Y remains/stays ...

This is a very common Italian sentence pattern and works much like English.