Vi gratulerer vinneren.

Because vinneren is the definite form of vinner.

• vinner = winner / a winner
• vinneren = the winner

In Norwegian, the definite article is often attached to the end of the noun instead of being a separate word in front.

So:

• Vi gratulerer vinneren. = We congratulate the winner.

Not We congratulate winner.

Vi means we.

It is the subject of the sentence, the people doing the action.

So the structure is:

• Vi = we
• gratulerer = congratulate / are congratulating
• vinneren = the winner

Gratulerer is the present tense of the verb å gratulere.

So:

• å gratulere = to congratulate
• gratulerer = congratulate / am congratulating / is congratulating / are congratulating, depending on the subject

Norwegian verbs do not change according to the subject the way English verbs sometimes do. The same present-tense form is used with all persons:

• jeg gratulerer
• du gratulerer
• vi gratulerer
• de gratulerer

Because Norwegian usually expresses definiteness by adding an ending to the noun.

So instead of a separate word like English the, Norwegian often uses a suffix:

• en vinner = a winner
• vinneren = the winner

That is why the is not written as a separate word here.

In Norwegian, å gratulere can take a direct object.

So Vi gratulerer vinneren is completely normal and means We congratulate the winner.

You can also often say what you are congratulating someone for, using med:

• Vi gratulerer vinneren med seieren. = We congratulate the winner on the victory.

So:

• gratulerer vinneren = congratulate the winner
• gratulerer vinneren med seieren = congratulate the winner on the victory

Yes. Norwegian present tense often covers both:

• We congratulate the winner
• We are congratulating the winner

Context usually tells you which meaning is intended.

So Vi gratulerer vinneren can be understood as either a general/simple present or an action happening now, depending on the situation.

Yes. This is the normal basic word order for a simple main clause:

• Subject + verb + object
• Vi + gratulerer + vinneren

If you move another element to the front, Norwegian usually keeps the verb in second position:

• I dag gratulerer vi vinneren. = Today we congratulate the winner.

That verb-second pattern is very important in Norwegian main clauses.

Usually not in the same basic meaning.

Vinneren already means the winner, so adding den is normally unnecessary unless you are making a more specific or contrastive expression, such as:

• den heldige vinneren = the lucky winner

In your sentence, the natural form is simply:

• Vi gratulerer vinneren.

The infinitive is å gratulere.

In dictionaries, Norwegian verbs are normally listed in the infinitive, often with å:

• å gratulere

So if you see gratulerer in a sentence and want to look it up, search for gratulere.

Yes. Gratulerer! is a very common standalone expression meaning Congratulations!

That is closely related to the full sentence:

• Vi gratulerer vinneren. = We congratulate the winner.
• Gratulerer! = Congratulations!

So learners often meet gratulerer both as a full verb in a sentence and as a fixed expression on its own.