Kondensvannet er under kjølerøret.

Because kondensvannet is the definite singular form of kondensvann.

In Norwegian, you usually do not put a separate word before the noun for the. Instead, the definiteness is often added to the end of the noun:

• kondensvann = condensation water / condensate
• kondensvannet = the condensation water

The ending -et is a common definite ending for neuter nouns.

Norwegian often expresses the by attaching an ending to the noun rather than using a separate article.

So:

• vann = water
• vannet = the water

And here:

• kondensvannet = the condensation water
• kjølerøret = the cooling pipe / the cooler pipe

This is one of the biggest differences from English.

Yes. Norwegian very often combines words into one compound noun.

Here:

• kondens = condensation
• vann = water

So kondensvann literally means condensation-water.

The same happens in:

• kjølerør = cooling pipe / cooler pipe
• kjøleskap = refrigerator
• sommerferie = summer holiday

If English would often write two words, Norwegian may prefer one compound word.

Kjølerøret is also a compound noun:

• kjøle = to cool
• rør = pipe / tube
• kjølerør = cooling pipe / cooler pipe
• kjølerøret = the cooling pipe / the cooler pipe

Depending on context, it could refer to a pipe connected to a cooling system, refrigerator, air conditioner, or similar equipment.

For the same reason as kondensvann becomes kondensvannet: it is the definite singular form.

• et kjølerør = a cooling pipe
• kjølerøret = the cooling pipe

The noun rør is neuter, so the definite singular ending is -et.

You usually learn a Norwegian noun together with its article:

• et kondensvann is not really how this word is commonly introduced, because kondensvann is often treated as a mass noun, but grammatically vann is neuter
• et rør = a pipe

A very useful habit is to memorize nouns with en, ei, or et:

• et rør
• et hus
• en bil

That helps you form the correct definite ending later:

• røret
• huset
• bilen

Yes. Under means under, beneath, or below, depending on context.

In this sentence, under kjølerøret means that the condensation water is located below the cooling pipe.

Examples:

• Boken ligger under bordet. = The book is under the table.
• Katten er under senga. = The cat is under the bed.

So this is a normal and straightforward use of under.

This is the normal basic word order for a simple statement in Norwegian:

subject + verb + place/location

So:

• Kondensvannet = subject
• er = verb
• under kjølerøret = location phrase

This is very similar to English:

• The condensation water
  • is
    • under the cooling pipe

Er is the present tense of å være = to be.

So:

• å være = to be
• er = is / are / am

In this sentence, er links the subject to its location:

• Kondensvannet er under kjølerøret.
• The condensation water is under the cooling pipe.

Yes, it is grammatical and understandable.

A native speaker might or might not choose exactly this wording depending on the situation. For example, in technical or practical contexts, kondensvann is perfectly normal. In everyday speech, someone might also say something simpler if the context is obvious.

But as a learning sentence, it is good Norwegian and shows useful patterns:

• compound nouns
• definite endings
• the verb er
• a location phrase with under

You would say:

• et kjølerør = a cooling pipe

Compare:

• et kjølerør = a cooling pipe
• kjølerøret = the cooling pipe

And similarly:

• kondensvann = condensation water
• kondensvannet = the condensation water

Those two parts are often hard for English speakers.

• kj in many dialects is a soft sound, somewhat like the h in hue for some English speakers, but not exactly. In some modern speech, it may merge with the sj sound, but learners are usually taught to aim for the traditional kj sound.
• ø is a rounded front vowel that English does not really have. You can think of it as somewhere between sounds in French or German rounded vowels, but it is best learned by listening and repeating.

Also note:

• røret has two r sounds and the vowel ø
• stress is typically on the first part of the compound: KJØ-le-rø-ret

The best approach is to listen to native audio and imitate it several times.