Matboden selger suppe.

Because Norwegian usually writes compound nouns as a single word. Here, mat + bod become matbod, meaning something like food stall or food booth. Then the definite ending is added, giving matboden.

A useful rule: in Norwegian, the last part of the compound is the main noun.

-en is the definite singular ending for many common-gender nouns in Bokmål.

So:

• en bod = a stall / a booth
• boden = the stall / the booth

That means matboden is the food stall.

It is capitalized only because it is the first word of the sentence.

In Norwegian, common nouns are normally not capitalized in the middle of a sentence. So if it came later, you would usually write matboden, not Matboden.

Because selger is the present tense form, while selge is the infinitive.

• å selge = to sell
• selger = sells / is selling

So in a full sentence, you need selger, not selge.

No. Norwegian verbs do not change for person the way English verbs do.

So you get:

• jeg selger = I sell
• du selger = you sell
• de selger = they sell

The verb form stays the same.

Because suppe is being used in a general sense, like a product the stall sells.

This works a lot like English:

• Matboden selger suppe. = The food stall sells soup.

You do not need en here when you mean soup in general.

Because suppe is being used like a mass noun, meaning soup in general, not individual soups.

If you said supper, that would sound more like:

• different kinds of soups, or
• soups as separate countable items

So selger suppe is the most natural way to say they sell soup in general.

Yes, in this sentence it is very similar:

• Matboden = subject
• selger = verb
• suppe = object

So the pattern is subject + verb + object, just like in English.

However, Norwegian has a strong verb-second rule in main clauses, so if another element comes first, the verb still stays in second position:

• I dag selger matboden suppe. = Today, the food stall sells soup.