U stakleniku je toplije nego vani, pa tamo držimo mlade sadnice.

Because u can take different cases depending on whether it means:

• location = in / inside → locative
• direction/movement into = into → accusative

Here, the sentence says in the greenhouse, so it uses the locative:

• staklenik = greenhouse
• u stakleniku = in the greenhouse

Compare:

• U stakleniku je toplo. = It is warm in the greenhouse.
• Idem u staklenik. = I am going into the greenhouse.

So -u on stakleniku marks the locative singular.

Toplije is the comparative form of toplo / topao and means warmer.

So:

• je toplo = it is warm
• je toplije = it is warmer

In this sentence, je is the 3rd person singular of biti (to be), so:

• U stakleniku je toplije = It is warmer in the greenhouse

Croatian often uses this impersonal structure with je for weather, temperature, and general conditions.

Croatian often leaves out subject pronouns, and in impersonal expressions it usually does not use a separate word corresponding to English it.

English says:

• It is warmer

Croatian simply says:

• Toplije je or Je toplije

This is normal in expressions about:

• weather
• temperature
• time
• general conditions

Examples:

• Hladno je. = It is cold.
• Kasno je. = It is late.
• Važno je. = It is important.

So the it is understood, not spoken.

Nego means than in comparisons.

So:

• toplije nego vani = warmer than outside

This is the standard way to make comparisons in Croatian:

• veći nego... = bigger than...
• brži nego... = faster than...
• bolji nego... = better than...

Examples:

• On je viši nego ja. = He is taller than I am.
• Ovdje je skuplje nego tamo. = It is more expensive here than there.

In your sentence, nego vani compares the temperature in the greenhouse with the temperature outside.

Vani is an adverb, meaning outside / outdoors.

So:

• nego vani = than outside

It does not change form for case, gender, or number.

Related words:

• vani = outside, outdoors
• unutra = inside
• ovdje = here
• tamo = there

You can also hear napolju in some varieties, but vani is very common and neutral.

Examples:

• Djeca su vani. = The children are outside.
• Hladno je vani. = It is cold outside.

Here pa means something like:

• so
• therefore
• and so

In this sentence:

• U stakleniku je toplije nego vani, pa tamo držimo mlade sadnice.
• It is warmer in the greenhouse than outside, so we keep young seedlings there.

It connects the two parts with a sense of consequence or result.

It is not exactly the same as plain and.
Compare:

• i = and
• pa = and so / so / then

Examples:

• Kasno je, pa idemo kući. = It is late, so we are going home.
• Pada kiša, pa ostajemo unutra. = It is raining, so we are staying inside.

Yes, they refer to the same place, but they do slightly different jobs.

• u stakleniku explicitly names the place: in the greenhouse
• tamo then refers back to that place: there

So the sentence works like this:

• In the greenhouse it is warmer than outside, so we keep young seedlings there.

This is very natural in Croatian, just as it is in English. It avoids repeating u stakleniku a second time.

You could also say:

• ...pa u stakleniku držimo mlade sadnice.

But tamo sounds more flowing and less repetitive.

Držimo is the 1st person plural present tense of the verb držati.

• držati = to hold, keep, maintain
• držimo = we hold / we keep

In this context, držati sadnice means to keep seedlings somewhere, not physically hold them in your hands.

A few forms of držati:

• držim = I keep
• držiš = you keep
• drži = he/she/it keeps
• držimo = we keep
• držite = you (plural/formal) keep
• drže = they keep

So:

• tamo držimo mlade sadnice = we keep young seedlings there

Mlade is the form of the adjective mlad (young) agreeing with sadnice (seedlings).

Since sadnice is:

• feminine plural
• here in the accusative plural (as the direct object)

the adjective must match it:

• mlade sadnice = young seedlings

Agreement in Croatian means adjectives match nouns in:

• gender
• number
• case

So the ending -e here reflects the form needed for feminine plural accusative, which for many feminine nouns looks the same as the nominative plural.

It is the accusative plural, because it is the direct object of držimo.

Ask the question:

• What do we keep?
• mlade sadnice

So:

• držimo sadnice = we keep seedlings

That requires the accusative.

With feminine nouns like sadnica, the accusative plural is:

• sadnice

And the adjective agrees:

• mlade sadnice

Singular/plural comparison:

• držimo mladu sadnicu = we keep a young seedling
• držimo mlade sadnice = we keep young seedlings

The dictionary form is sadnica.

So:

• sadnica = seedling / young plant
• sadnice = seedlings

It is a feminine noun.

Basic forms:

• sadnica = singular
• sadnice = plural

In gardening vocabulary, sadnica is very common for a plant raised for planting out later.

Croatian word order is fairly flexible, but the original sentence is very natural and neutral:

• U stakleniku je toplije nego vani, pa tamo držimo mlade sadnice.

You could rearrange parts for emphasis, for example:

• Toplije je u stakleniku nego vani, pa tamo držimo mlade sadnice.
• Mlade sadnice tamo držimo, jer je u stakleniku toplije nego vani.

But not every order sounds equally neutral. The original version is good because it:

1. sets the location first
2. states the comparison
3. gives the result

That makes it easy to follow.

Usually not in the same way.

• vani = outside, outdoors → adverb
• izvan = outside of, beyond → preposition

So in your sentence, vani works naturally because it stands alone:

• nego vani = than outside

But izvan normally needs a noun phrase after it:

• izvan kuće = outside the house
• izvan grada = outside the city

So:

• toplije nego vani = correct
• toplije nego izvan = not natural/incomplete

Yes. Staklenik is a masculine noun.

You can tell from its dictionary form ending and from the locative singular form here:

• staklenik = greenhouse
• u stakleniku = in the greenhouse

Other masculine nouns behave similarly:

• vrt → u vrtu
• grad → u gradu
• podrum → u podrumu

So if you are learning case endings, staklenik is a useful example of a regular masculine inanimate noun.