Drivhuset er varmt.

Because drivhuset is the definite form of the noun: the greenhouse.

• et drivhus = a greenhouse
• drivhuset = the greenhouse

In Norwegian, the definite article is usually added to the end of the noun as a suffix. For neuter nouns like drivhus, that ending is -et.

Er is the present tense of å være (to be).

So:

• er = is / are / am, depending on context

Norwegian does not change the verb for different persons the way English does. So you get:

• jeg er = I am
• du er = you are
• han er = he is
• vi er = we are

It stays er for all of them.

Because drivhus is a neuter noun.

When an adjective comes after er and describes a singular noun, it usually agrees with the noun’s gender:

• common gender: varm
• neuter: varmt
• plural / definite plural contexts: varme

So:

• En stol er varm. = A chair is warm.
• Et drivhus er varmt. = A greenhouse is warm.

Since drivhus is neuter, varm becomes varmt.

You learn the gender together with the noun:

• et drivhus

A very good habit is to memorize Norwegian nouns with their article:

• et drivhus
• en bil
• ei/en bok

Also, drivhus is a compound noun, and in Norwegian the last part usually decides the gender. The last part here is hus, and et hus is neuter, so et drivhus is neuter too.

It is a very basic Norwegian main clause:

subject + verb + adjective

• Drivhuset = subject
• er = verb
• varmt = predicate adjective

This is the normal word order in a simple statement.

In a simple statement like this, yes: subject + verb + rest.

But Norwegian is a V2 language, which means the verb usually comes in the second position in main clauses. If you start with another element, the verb still stays second:

• Drivhuset er varmt.
• I dag er drivhuset varmt. = Today the greenhouse is warm.

Notice that after I dag, the verb er comes before drivhuset.

Yes, and it is very natural, but the focus is slightly different.

• Drivhuset er varmt. = The greenhouse is warm.
  Focus: the greenhouse itself as the subject.

• Det er varmt i drivhuset. = It is warm in the greenhouse.
  Focus: the temperature inside it.

Both are correct, but they are not used in exactly the same way.

Because Norwegian usually puts the definite article at the end of the noun instead of using a separate word before it.

So:

• the greenhouse = drivhuset
• not a separate word like English the

This is one of the big differences between English and Norwegian nouns.

It can cover both warm and sometimes hot, depending on context, but warm is usually the safest basic meaning here.

If you want to be clearly stronger, Norwegian often uses:

• hett = hot

So:

• Drivhuset er varmt often suggests the greenhouse is warm
• Drivhuset er hett would sound more like the greenhouse is hot

Context matters.

A simple learner-friendly guide would be:

DREEV-hoo-set er varmt

A few helpful points:

• drivhuset has the stress mainly on the first part: DRIV-
• hus sounds like hoos, but with a Norwegian u sound
• the t in varmt may be light or not strongly released in natural speech

The exact pronunciation varies a bit by dialect, but this is a good starting point.