Kad grijanje radi dobro, mačka spava pokraj radijatora, a ja sjedim u fotelji.

Kad means when.

Yes, it is basically the same as kada.

• kad = shorter, very common in everyday speech
• kada = fuller form, also correct and often a bit more formal or emphatic

So Kad grijanje radi dobro... means When the heating works well...

Yes. The verb raditi often means to work in the sense of to function.

So:

• grijanje radi = the heating works / is working
• radi dobro = works well

That is a very natural Croatian way to say that a system, machine, or device is functioning properly.

Here grijanje is a noun, meaning heating.

It comes from the verb grijati (to heat), but in this sentence it functions as a thing:

• grijanje = heating, the heating system

It is neuter singular, which is why the verb is:

• grijanje radi
  not
• grijanje rade

Because dobro here is an adverb, not an adjective.

It modifies the verb radi:

• radi dobro = works well

Compare:

• dobar radijator = a good radiator → adjective describing a noun
• radijator radi dobro = the radiator works well → adverb describing a verb

English also does this:

• good radiator
• works well

Because mačka is the subject of the verb spava.

The subject is in the nominative case:

• mačka = the cat

So:

• mačka spava = the cat is sleeping / sleeps

If mačka were an object, another case might be used, but here it is the thing doing the action.

Pokraj takes the genitive case.

So:

• radijator = nominative
• radijatora = genitive singular

That is why you get:

• pokraj radijatora = next to / beside the radiator

This is a very important pattern to remember:

• pokraj + genitive

Other examples:

• pokraj kuće = beside the house
• pokraj prozora = beside the window

They are very close in meaning. Both can mean beside, next to, or by.

So:

• pokraj radijatora
• pored radijatora

Both are natural and correct in many contexts.

There can be small style or regional preferences, but for a learner it is safe to understand them as near-equivalents here.

This is a very common question.

Both a and i can connect parts of a sentence, but they are not the same.

• i = and, simple addition
• a = and / while / whereas / but, often showing contrast or a shift of topic

In this sentence:

• mačka spava pokraj radijatora, a ja sjedim u fotelji

The idea is:

• the cat sleeps by the radiator, while I sit in the armchair

So a works well because it gently contrasts the cat with I.

Yes, Croatian could simply say:

• ..., a sjedim u fotelji.

Because the verb sjedim already tells you the subject is I.

Croatian often drops subject pronouns when they are clear from the verb ending.

But ja is included here for emphasis or contrast:

• the cat does one thing, I do another

So a ja feels natural because it highlights the difference:

• the cat sleeps..., and I sit...

It can mean both, depending on context.

• sjedim = I sit
• sjedim = I am sitting

Croatian present tense often covers both:

• a general/habitual meaning
• an action happening now

So this sentence could mean:

• When the heating works well, the cat sleeps by the radiator, and I sit in the armchair. or
• When the heating is working well, the cat is sleeping by the radiator, and I am sitting in the armchair.

The exact interpretation depends on context.

Because after u meaning in, Croatian usually uses the locative case when talking about location.

So:

• fotelja = nominative
• u fotelji = in the armchair

This is the location pattern:

• u + locative = in/at a place

Compare:

• u kući = in the house
• u sobi = in the room
• u fotelji = in the armchair

Note that u + accusative is used when there is movement into something:

• Sjedam u fotelju. = I am sitting down into the armchair.
• Sjedim u fotelji. = I am sitting in the armchair.

Not exactly.

• fotelja = armchair, a big comfortable chair
• stolica = chair, more general
• naslonjač can also mean armchair, depending on context and region

So u fotelji gives a more cozy image than na stolici.

Because Croatian often uses the present tense for things that are:

• happening now
• generally true
• habitual or repeated

In this sentence, the present tense creates a natural scene or routine:

• Kad grijanje radi dobro...
• mačka spava...
• ja sjedim...

This can describe:

1. a general pattern: Whenever the heating works well...
2. a current situation: When the heating is working well...

Croatian does not need a special continuous form like English is sleeping or am sitting.

The word order can change more easily in Croatian than in English, although the original sentence is very natural.

For example, you might also hear:

• Kad grijanje dobro radi, mačka spava pokraj radijatora, a ja sjedim u fotelji.

This is also correct. The difference is mostly one of emphasis and style.

The original version:

• Kad grijanje radi dobro...

is straightforward and neutral.

Croatian word order is flexible, but not random. It often changes to emphasize a certain word or to sound more natural in context.

Croatian has no articles like a, an, or the.

So:

• mačka can mean a cat or the cat
• radijatora can mean a radiator or the radiator
• u fotelji can mean in an armchair or in the armchair

The exact meaning comes from context.

In this sentence, English will often naturally use the:

• the cat
• the radiator
• the armchair

But Croatian itself does not mark that with an article.