Den upplevelsen glömmer jag aldrig.

Because upplevelse is an en-word (common gender) in Swedish.

• en upplevelse = an experience
• den upplevelsen = that experience / that particular experience

You use den with en-words and det with ett-words.

The ending -en marks the definite singular form of the noun.

• upplevelse = an experience
• upplevelsen = the experience

So the noun itself already carries definiteness. In this sentence, den upplevelsen is not just a plain the experience; it has a more pointed meaning, like that experience or that particular experience.

It can look that way at first, but here den is acting more like a demonstrative: that.

So:

• upplevelsen = the experience
• den upplevelsen = that experience / that particular experience

This is a common Swedish pattern when you want to point something out more strongly or contrast it with other things.

Because Swedish often moves an element to the front for emphasis or to make it the topic of the sentence.

Compare:

• Jag glömmer aldrig den upplevelsen. = neutral word order
• Den upplevelsen glömmer jag aldrig. = emphasis on that experience

It is similar to English That experience I’ll never forget, which sounds more marked than the neutral I’ll never forget that experience.

Because Swedish main clauses follow the V2 rule: the finite verb must come in the second position.

Here, Den upplevelsen is placed first. That means the finite verb glömmer must come second, and the subject jag comes after it:

• Den upplevelsen | glömmer | jag | aldrig

This is a very important Swedish word-order pattern.

Swedish very often uses the present tense where English uses will.

So glömmer jag aldrig literally looks like I never forget, but in natural English it is often translated as I will never forget.

The Swedish present tense can express:

• a general truth
• a firm future meaning
• something the speaker feels is permanently true

Yes. That is also correct and very natural.

• Jag kommer aldrig att glömma den upplevelsen = I will never forget that experience
• Den upplevelsen glömmer jag aldrig = That experience I’ll never forget

The version with glömmer is shorter and punchier. The version with kommer att glömma is a more explicit future form.

Aldrig is a sentence adverb, and its position depends on the sentence structure.

In a neutral main clause:

• Jag glömmer aldrig den upplevelsen.

When something else is moved to the front, Swedish keeps the verb second, then the subject, and then aldrig often comes after the subject:

• Den upplevelsen glömmer jag aldrig.

So the placement is a result of normal Swedish word order, not something unusual.

Yes, usually it is.

Den upplevelsen glömmer jag aldrig sounds more emphatic, memorable, and slightly more dramatic. It highlights that experience as the important part.

So the difference is mostly about focus:

• Jag glömmer aldrig den upplevelsen = neutral
• Den upplevelsen glömmer jag aldrig = emphasis on the experience

Here are the main forms:

• en upplevelse = an experience
• upplevelsen = the experience
• upplevelser = experiences
• upplevelserna = the experiences

So it is a regular en-word noun.