Selv når frostnatten er kald, står plantene i drivhuset.

Selv når means even when.

• når = when
• selv adds the idea of even

So selv når introduces a situation that might seem difficult or surprising, but the main clause is still true.

Yes, you often could, but the nuance is a little different.

• selv når = even when
  This sounds more temporal or general/repeated.
• selv om = even though / although
  This sounds more like a contrast or concession.

In this sentence, selv når frostnatten er kald suggests something like whenever the frosty night is cold or even in those cold frosty nights.
If you said selv om frostnatten er kald, the meaning would be closer to even though the frosty night is cold.

Both can work, but selv når fits well if the idea is a general condition.

This is because Norwegian main clauses normally follow the V2 rule: the finite verb comes in the second position.

Here, the sentence begins with the subordinate clause:

Selv når frostnatten er kald

That whole clause counts as the first element. So in the main clause, the verb must come next:

• står = second position
• plantene = subject after the verb

So:

• Selv når frostnatten er kald, står plantene i drivhuset.

If the sentence started directly with the subject, then you would get:

• Plantene står i drivhuset.

In Norwegian, the definite article is often attached to the end of the noun.

So:

• frostnatten = the frosty night / the frost night
• plantene = the plants
• drivhuset = the greenhouse

The endings are doing the job of English the:

• -en in frostnatten
• -ene in plantene
• -et in drivhuset

This is very common in Norwegian.

It is a compound noun, which is extremely common in Norwegian.

It is built from:

• frost = frost
• natt = night

Together: frostnatt = frost night / frosty night

Then the definite ending is added:

• frostnatten = the frost night

Norwegian usually writes compound nouns as one word, much more often than English does.

Because kald agrees with a singular common-gender noun used as a predicate adjective.

Here the noun is frostnatten, and natt is treated here with the definite form -en. So the adjective form is:

• kald for singular common gender

Compare:

• Natten er kald. = The night is cold.
• Huset er kaldt. = The house is cold.
  (kaldt because hus is neuter)
• Nettene er kalde. = The nights are cold.
  (kalde because plural)

So kald is the correct form here.

Because Norwegian often uses verbs like stå, ligge, and sitte where English would simply use be.

Here står suggests that the plants are standing / positioned in the greenhouse. That sounds natural in Norwegian.

Compare:

• Plantene står i drivhuset.
  The plants are in the greenhouse / are standing in the greenhouse.
• Plantene er i drivhuset.
  The plants are in the greenhouse.

Both can be grammatical, but står is often more idiomatic when talking about plants or objects that are upright or placed somewhere.

Plantene is the definite plural: the plants.

• planter = plants
• plantene = the plants

The sentence is talking about a specific set of plants, not plants in general.

If you said Planter står i drivhuset, that would sound more like a general statement about plants as a category, and it would not fit this sentence as naturally.

Because the sentence begins with a subordinate clause, and in Norwegian it is standard to put a comma before the following main clause.

So:

• Selv når frostnatten er kald, står plantene i drivhuset.

The comma helps mark the boundary between:

1. the subordinate clause: Selv når frostnatten er kald
2. the main clause: står plantene i drivhuset

This is a very common pattern in Norwegian writing.

Yes, in Bokmål that is possible.

Many nouns that are historically feminine can appear in two ways in Bokmål:

• natten
• natta

So you may also see:

• frostnatta

Both are acceptable in Bokmål, depending on style and dialect preference.
Using frostnatten is a bit more conservative or neutral in many contexts, while frostnatta can feel more natural or colloquial for some speakers.

The preposition i means in or inside.

So i drivhuset means in the greenhouse.

That makes sense because the plants are physically located inside it.

Also notice again the definite ending:

• drivhus = greenhouse
• drivhuset = the greenhouse

If it were indefinite, you could say:

• i et drivhus = in a greenhouse