Sopparten har et kjennetegn på undersiden.

Sopparten means the mushroom species or the fungal species, depending on context.

It is made up of:

• sopp = mushroom / fungus
• art = species

Together, soppart means mushroom species or fungal species.
The ending -en makes it definite, so sopparten = the mushroom species.

Because Norwegian very often forms compound nouns as a single word.

So:

• sopp
  • art = soppart

This is very normal in Norwegian. English often uses two words where Norwegian uses one:

• mushroom species → soppart
• car door → bildør
• school book → skolebok

Har means has, while er means is / are.

In this sentence, the idea is that the species has a distinguishing feature on its underside. So har is the correct verb:

• Sopparten har et kjennetegn ... = The mushroom species has a distinguishing feature ...

If you used er, the meaning would change completely.

Kjennetegn means distinguishing feature, characteristic, or identifying mark.

It is often used for something that helps you recognize or identify something.

For example:

• Et kjennetegn ved arten ... = A distinguishing feature of the species ...
• Dette er et viktig kjennetegn. = This is an important identifying feature.

Because kjennetegn is a neuter noun in Norwegian, so it takes et in the indefinite singular.

• et kjennetegn = a distinguishing feature
• kjennetegnet = the distinguishing feature

This is something learners often just have to memorize noun by noun.

Because the sentence says a distinguishing feature, not the distinguishing feature.

• et kjennetegn = a distinguishing feature / one identifying feature
• kjennetegnet = the distinguishing feature

Using the indefinite form suggests that this is one feature among possibly several, or that it has not been specifically identified yet.

På undersiden means on the underside or on the lower side.

It refers to the bottom surface of something.

In this sentence, it means the identifying feature is located on the underside of the mushroom or fungus.

Because undersiden is the definite form: the underside.

• underside = underside
• undersiden = the underside

Norwegian often uses the definite form where English might also use the, especially when the part of something is understood from context.

So:

• på undersiden = on the underside

Yes, it is a very natural and common way to say on the underside in Norwegian.

You could think of it as:

• på = on
• undersiden = the underside

Norwegian often uses på with surfaces and sides:

• på toppen = on the top
• på siden = on the side
• på undersiden = on the underside

The sentence follows normal Norwegian word order:

• Sopparten = subject
• har = verb
• et kjennetegn = object
• på undersiden = prepositional phrase telling where

So the pattern is:

Subject + verb + object + place/location phrase

That is very similar to English:

• The mushroom species
  • has
    • a distinguishing feature
      • on the underside

Yes, it is grammatical and understandable, especially in a descriptive or scientific context.

It sounds like something from a biology text, field guide, or species description.

In everyday speech, a speaker might sometimes make it more specific, for example:

• Denne sopparten har et kjennetegn på undersiden. = This mushroom species has a distinguishing feature on the underside.

But the original sentence is perfectly fine.

Yes. Sopp can refer broadly to fungus/fungi, but in many contexts it is also used for mushroom.

So soppart may be understood as:

• mushroom species
• fungal species

The best translation depends on context. If the sentence is about visible mushrooms in a field guide, mushroom species is often the most natural English choice.