Learn Hardcore Croatian: Zimi često ima snijega, a nebo je puno oblaka. - In winter there is often snow, and the sky is full of clouds.

You can see u zimu in some contexts, but it sounds more marked, specific, or stylistic. For everyday usage, learn zimi as the default for “in winter”.

In ima snijega, why is snijega in the genitive, and what is the subject here?

The verb imati (ima) normally means “to have”, but in this pattern it works more like English “there is / there are”.

ima + GENITIVE = there is / there are (some amount of X)

So:

ima snijega = there is (some) snow
- snijeg → snijega (genitive singular)

Here:

There is no explicit subject like English “there”. Croatian doesn’t need a dummy subject.
snijega is in the genitive because after ima in this existential meaning, you normally use the genitive to indicate an indefinite amount (“some snow”, not a specific block of snow).

Compare:

Ima snijega. – There is (some) snow.
Nema snijega. – There isn’t any snow.

The same works with countable nouns:

Ima automobila na cesti. – There are (some) cars on the road. (automobila = genitive plural)

Can I say Pada snijeg instead of Ima snijega? Do they mean the same thing?

They are related, but not the same:

Pada snijeg. – Snow is falling. / It’s snowing (now / at this moment).
Ima snijega. – There is (some) snow. (e.g. on the ground, in general in winter)

So:

Use pada snijeg to talk about the action of snow falling.
Use ima snijega to talk about presence/amount of snow, not necessarily that it is falling right now.

In your sentence, Zimi često ima snijega…, the idea is “In winter there is often snow (around)”, not specifically “it is often snowing”.

What exactly does ima mean here? I thought imati = “to have”.

You’re right: imati usually means “to have” (Ja imam knjigu – I have a book).

But in sentences like Ima snijega, ima is used impersonally, with the meaning:

ima + GENITIVE = there is / there are (some …)

So:

Ima snijega. – There is (some) snow.
Ima ljudi u parku. – There are (some) people in the park.
Nema kruha. – There is no bread.

Think of this as a special construction you should memorize, separate from the normal “X ima Y = X has Y”.

What does the conjunction a do here? Is it just “and”?

In this sentence:

Zimi često ima snijega, a nebo je puno oblaka.

a is usually translated as “and”, but it often carries a nuance of:

contrast, mild opposition, or topic shift.

It’s weaker than “but”, but not as neutral as a simple listing i = and.

Very roughly:

i = and (simply adds things)
a = and / while / whereas (adds, but also contrasts or shifts focus)

Here it links two related winter facts, with a slight “and also / while at the same time” feeling:

“In winter there is often snow, and (at the same time) the sky is full of clouds.”

Why is it nebo je puno oblaka and not nebo je puno oblaci?

Because puno is followed by a genitive noun:

puno + GENITIVE = a lot of / many / much X

The noun oblak (“cloud”) has:

Nominative plural: oblaci – clouds (subject form)
Genitive plural: oblaka – of clouds

So:

puno oblaka = a lot of clouds / many clouds
nebo je puno oblaka = the sky is full of clouds / there are many clouds in the sky

Using oblaci here would be incorrect, because after puno you need genitive, not nominative.

What is puno here – an adjective (“full”) or a quantifier (“a lot of”)?

In nebo je puno oblaka, puno behaves like a quantifier meaning “a lot (of)” and it takes a genitive noun:

puno oblaka – a lot of clouds
puno ljudi – a lot of people
puno posla – a lot of work

You can treat puno + GENITIVE as a very common fixed pattern meaning “a lot of …”.

There is a related adjective pun, -a, -o (“full”), which agrees with the noun:

Čaša je puna vode. – The glass is full of water.
Džep je pun novčića. – The pocket is full of coins.

In everyday speech, the quantifier use puno + GENITIVE is much more frequent and very productive, so it’s good to learn that pattern first.

Could I also say Nebo je oblačno instead of Nebo je puno oblaka?

Yes, but the nuance is a bit different:

Nebo je oblačno. – The sky is cloudy. (general weather description)
Nebo je puno oblaka. – The sky is full of clouds / There are many clouds in the sky. (emphasizes the amount of individual clouds)

Both are correct; the original sentence chooses puno oblaka to stress “a lot of clouds” rather than just “cloudy weather”.

Where should the adverb često go? Is Zimi ima često snijega correct?

In your sentence, the most natural position is:

Zimi često ima snijega… – In winter there is often snow…

Other natural options:

Često zimi ima snijega… – Often, in winter there is snow…
(more neutral) Zimi ima dosta snijega. – In winter there is quite a lot of snow.

Zimi ima često snijega is not wrong, but it sounds less natural and a bit clunky. As a rule of thumb:

Put adverbs like često, uvijek, ponekad, rijetko before the main verb, or
After an initial time phrase: Zimi često…, Ljeti obično…, etc.

Why is je placed after nebo in nebo je puno oblaka? Could I say je nebo puno oblaka?

In normal statements, Croatian clitic verbs like je (is) usually stand in second position in the clause:

Nebo je puno oblaka. – The sky is full of clouds.

So the order is:

First stressed word: Nebo
Then the clitic: je
Then the rest: puno oblaka

You normally cannot say *Je nebo puno oblaka. as a neutral statement. You would only move je in certain question or emphasis structures:

Je li nebo puno oblaka? – Is the sky full of clouds?

For ordinary declarative sentences, keep je after the first word or phrase of the clause.