Melken er sur.

Because melken is the definite form: it means the milk.

In Norwegian, you usually make a noun definite by adding the article to the end of the noun instead of putting a separate word before it.

• melk = milk
• melken = the milk

So Melken er sur is talking about a specific milk, for example the milk in the fridge.

The ending -en is the definite singular ending here. It does the job that English the does.

So:

• melk = milk
• melken = the milk

This is a very common Norwegian pattern:

• bil → bilen
• kopp → koppen
• hund → hunden

Yes. Melka is also a common and correct form in Bokmål.

Many speakers treat melk as a feminine noun, so the definite form can be:

• melka
• melken

Both are used. Melka may sound more everyday or dialect-friendly to some speakers, while melken is also fully standard.

Because the adjective agrees with the subject.

Here, melken is singular and non-neuter, so the adjective stays in its basic form: sur.

Compare:

• Melken er sur.
• Ølet er surt. → neuter singular
• Sitronene er sure. → plural

So sur is the correct form for a singular non-neuter subject.

Because sur is used predicatively here, meaning it comes after er and describes the subject.

In Norwegian, adjectives often behave differently depending on where they are:

• den sure melken = the sour milk
  
  • adjective comes before the noun
• melken er sur = the milk is sour
  
  • adjective comes after er

So even though melken is definite, you still say sur, not sure, because the adjective is not directly in front of the noun.

Yes, in this sentence it is very similar to English:

• Melken = subject
• er = verb
• sur = adjective

So it follows:

Subject + verb + adjective

Just like The milk is sour.

Yes. Sur has more than one common use.

With food or drink, it means something like:

• sour
• gone sour
• sometimes spoiled, depending on context

With people, it usually means:

• grumpy
• sulky
• in a bad mood
• sometimes angry

So:

• Melken er sur. = the milk is sour
• Han er sur. = he is grumpy / in a bad mood

The context tells you which meaning is intended.

Melken er sur describes the milk’s condition right now.

Melken blir sur means the milk is becoming sour or goes sour.

So:

• Melken er sur. = The milk is sour.
• Melken blir sur. = The milk is becoming sour / goes sour.

If you want to say the change has already happened, you can also say:

• Melken har blitt sur. = The milk has gone sour.