Kad u listopadu naglo zahladi, uvijek izvadim deblji kaput iz ormara.

Both kad and kada mean when.

In this sentence, kad is the normal, everyday choice.
kada is a bit fuller and can sound slightly more formal or more careful in style, but the meaning is the same here.

So:

• Kad u listopadu naglo zahladi...
• Kada u listopadu naglo zahladi...

Both are correct.

Because u meaning in takes the locative case when it refers to location or time.

Here, listopad is the name of the month October, and after u it becomes listopadu:

• nominative: listopad
• locative: u listopadu

So u listopadu means in October.

A useful comparison:

• u Zagrebu = in Zagreb
• u kući = in the house
• u listopadu = in October

In Croatian, names of months are normally written with a lowercase letter.

So:

• listopad = October
• studeni = November
• siječanj = January

This is different from English, where month names are capitalized.

naglo is an adverb meaning suddenly, sharply, or abruptly.

zahladi is a verb form meaning it gets cold / it turns cold.

This is a common kind of impersonal weather expression in Croatian. English uses a dummy subject it in sentences like it gets cold, but Croatian usually just uses the verb:

• Zahladilo je. = It got cold.
• Kad zahladi... = When it gets cold...

So there is no separate word for it here.

This is mostly about aspect.

Both zahladi and izvadim are perfective verbs/forms. In Croatian, the present forms of perfective verbs usually do not describe something happening right now in an ongoing way. Instead, they often refer to:

• a future complete event, or
• a repeated complete event in a habitual situation

So:

• kad zahladi = when it gets cold
• izvadim = I take out / I will take out in the sense of one complete action

The sentence describes a repeated seasonal pattern: whenever that event happens, this other event happens.

Because uvijek means always, and it can refer to something that happens repeatedly as a complete action each time.

That is exactly what the perfective verb does here:

• each time it suddenly gets cold,
• I take out the coat as one finished action

So uvijek izvadim does not mean one continuous ongoing action. It means:

• every time the situation occurs,
• I do that complete action

This is very natural in Croatian.

Literally, yes: deblji is the comparative of debeo, so it means thicker.

But in natural English, with clothing, this often means something like:

• a thicker coat
• a heavier coat
• a warmer coat

It often suggests a comparison with a lighter coat.

Also, Croatian often uses the comparative in a way that can imply the thicker one when the context is clear.

So deblji kaput can mean either:

• a coat that is thicker than another one, or
• the thicker/heavier coat I use when it gets colder

Because kaput is the direct object of izvadim, so it is in the accusative case.

However, masculine inanimate nouns in Croatian often have the same form in nominative and accusative singular.

So:

• nominative: kaput
• accusative: kaput

That is why the form does not visibly change.

Compare this with a masculine animate noun:

• pas = dog
• nominative: pas
• accusative: psa

So:

• Vidim psa. = I see the dog.
• Izvadim kaput. = I take out the coat.

Because the preposition iz means out of / from, and it requires the genitive case.

The noun ormar becomes ormara in the genitive singular:

• nominative: ormar
• genitive: ormara

So:

• iz ormara = out of the wardrobe / from the closet

This is a very common pattern:

• iz kuće = out of the house
• iz škole = from school
• iz džepa = out of the pocket

Because Croatian is a pro-drop language.

The verb ending already tells you the subject:

• izvadim = I take out

The ending -im here shows first person singular, so ja is unnecessary.

Croatian usually includes ja only for emphasis, contrast, or clarity:

• Ja uvijek izvadim deblji kaput, a on ne. = I always take out the thicker coat, but he doesn’t.

Without that emphasis, leaving out the pronoun is more natural.

Croatian has no articles like English a/an/the.

So deblji kaput can mean:

• a thicker coat
• the thicker coat

The exact meaning depends on context.

If Croatian wants to be more specific, it can use other words, for example:

• taj deblji kaput = that thicker coat
• jedan deblji kaput = a thicker coat / one thicker coat

But in many ordinary sentences, no article-like word is needed.

Croatian word order is fairly flexible, though some orders sound more neutral than others.

The original sentence is very natural and neutral:

• Kad u listopadu naglo zahladi, uvijek izvadim deblji kaput iz ormara.

You could also hear variations like:

• U listopadu, kad naglo zahladi, uvijek izvadim deblji kaput iz ormara.
• Kad u listopadu naglo zahladi, deblji kaput uvijek izvadim iz ormara.

These are possible, but the emphasis changes slightly.

The original order is a good standard pattern:

1. time clause first: Kad u listopadu naglo zahladi
2. main clause after the comma: uvijek izvadim deblji kaput iz ormara

Because the sentence begins with a subordinate clause introduced by kad:

• Kad u listopadu naglo zahladi = subordinate time clause

Then comes the main clause:

• uvijek izvadim deblji kaput iz ormara

In Croatian, when a subordinate clause comes before the main clause, it is normally separated by a comma.

So the comma here is standard and expected.