Middagen blir festlig.

Because -en marks the noun as definite singular.

• middag = dinner / a dinner
• middagen = the dinner

In Norwegian, the definite article is often attached to the end of the noun instead of being a separate word. So English the dinner becomes middagen.

Blir is the present tense of å bli.

The verb å bli often means:

• to become
• to get
• sometimes to be / will be in a future-like sense

In this sentence, blir suggests something like will be, is going to be, or turns out to be, depending on context.

Useful forms:

• å bli = infinitive
• blir = present
• ble = past
• blitt = past participle

There is a small difference in nuance:

• Middagen er festlig = the dinner is festive
• Middagen blir festlig = the dinner will be / is becoming / is turning out festive

So er is more static, while blir suggests development, result, or a future situation.

Yes. Norwegian often uses the present tense to talk about the future when the context makes it clear.

So a present-tense verb can sound future-like in English.

Here, blir is grammatically present tense, but the natural English translation may still be will be festive.

Because it is a predicate adjective after the verb blir.

In this sentence:

• middagen = subject
• blir = linking verb
• festlig = predicate adjective

With predicate adjectives, Norwegian still shows agreement, but in the singular this adjective is festlig. The plural form is festlige.

For example:

• Middagen blir festlig = singular
• Middagene blir festlige = plural

Also, adjectives ending in -ig often keep the same form in singular.

Usually, in modern Norwegian, middag means the main cooked meal of the day, often what English speakers call dinner.

But meal words can vary by region, habit, and family. In some contexts, what counts as middag depends on when the main meal is eaten.

So middag is usually best understood as dinner or the main meal, not necessarily a strict clock-time word.

Because Norwegian normally puts definiteness on the noun itself.

So:

• en middag = a dinner
• middagen = the dinner

A separate determiner is used in some other structures, especially with adjectives before the noun:

• den festlige middagen = the festive dinner

That is why middagen by itself already includes the idea of the.

Yes, potentially.

That is one reason bli can be tricky for English speakers. Depending on context, blir can point to:

• a future result
• a change of state
• how something is shaping up

So Middagen blir festlig could mean:

• the dinner will be festive
• the dinner is turning festive
• the dinner is shaping up to be festive

The exact nuance depends on the situation around the sentence.