Bananen var ikke moden i går, men eplet var godt.

Because Norwegian is referring to the banana and the apple, not just a banana and an apple.

• en banan = a banana
• bananen = the banana
• et eple = an apple
• eplet = the apple

In a sentence like this, Norwegian often uses the definite form when talking about specific things already known from the situation or context.

Because adjectives in Norwegian agree with the noun’s gender and sometimes number.

• banan is a common gender noun: en banan
• eple is a neuter noun: et eple

So in the singular:

• common gender: moden
• neuter: godt

That is why you get:

• Bananen var ikke moden
• eplet var godt

This is normal adjective agreement after å være as well.

Because these adjectives are being used predicatively, not attributively.

In other words, they come after the verb var and describe the subject:

• Bananen var ikke moden
• eplet var godt

When an adjective comes after a verb like er / var / blir, it usually agrees only for gender and number, not for definiteness.

Compare:

• det gode eplet = the good apple
  
  • here the adjective is before the noun, so it takes the definite form gode
• eplet var godt = the apple was good
  
  • here the adjective is after the verb, so it is godt

Because in a normal Norwegian main clause, the finite verb usually comes in second position, and ikke usually comes after that verb.

So:

• Bananen = first element
• var = finite verb in second position
• ikke = negation after the verb

That gives:

• Bananen var ikke moden i går

This is a very common pattern in Norwegian main clauses.

I går means yesterday.

Yes, it can move, depending on emphasis. For example:

• Bananen var ikke moden i går, men eplet var godt.
• I går var bananen ikke moden, men eplet var godt.

Both are grammatical, but the first version sounds more neutral if you are simply adding the time information. The second puts more focus on yesterday.

Because there are two separate clauses joined by men:

1. Bananen var ikke moden i går
2. men eplet var godt

In Norwegian, it is completely normal to repeat the verb in the second clause. This is the natural way to say it.

Men means but.

It connects two ideas that contrast with each other:

• the banana was not ripe
• but the apple was good

So men introduces a contrast, just like but in English.

Because the sentence is talking about the past.

• er = is
• var = was

Since the sentence includes i går (yesterday), the past tense is needed:

• Bananen var ikke moden i går

Using er would not match i går.

It can mean both, depending on context. With food, god/godt very often means:

• good
• nice
• tasty

So eplet var godt will usually be understood as the apple tasted good or the apple was nice/tasty.

Because Norwegian, like English, often leaves out repeated information when it is already clear.

The sentence says:

• Bananen var ikke moden i går, men eplet var godt.

The time phrase i går naturally carries over in meaning unless the context suggests otherwise. So it is understood as something like:

• The banana wasn’t ripe yesterday, but the apple was good.

If you wanted to be extra explicit, you could repeat i går, but it is usually unnecessary.

Here are the basic forms:

• bananen → en banan
• var → past tense of å være
• ikke → not
• moden → ripe
• i går → yesterday
• men → but
• eplet → et eple
• godt → adjective from god

So the sentence gives you examples of:

• definite nouns
• the past tense of å være
• negation
• adjective agreement