Det føles næsten, som om sommeren allerede er begyndt, når vi spiser jordbær på terrassen efter en varm dag.

Here det is a dummy subject, just like it in English It feels almost as if...

Danish often uses det in general statements about feelings, weather, time, and impressions:

• Det regner = It is raining
• Det virker mærkeligt = It seems strange
• Det føles godt = It feels good

So in Det føles næsten..., det does not refer to a specific thing. It simply introduces the experience or impression.

Because føles means feels in the sense of seems / gives a feeling, while føler usually means feel in the sense of personally sensing or experiencing something.

Compare:

• Jeg føler mig træt = I feel tired
• Det føles som sommer = It feels like summer

So Det føles næsten... is the natural way to say It almost feels...

Som om means as if.

It introduces something that is being compared to a feeling, impression, or appearance:

• Det ser ud, som om det regner = It looks as if it is raining
• Det lyder, som om hun er glad = It sounds as if she is happy

In this sentence:

• Det føles næsten, som om sommeren allerede er begyndt
  = It almost feels as if summer has already begun

You should learn som om as a fixed expression.

This is because Danish changes word order in subordinate clauses.

In a main clause, the finite verb usually comes early:

• Sommeren er allerede begyndt = Summer has already begun

But after a subordinating expression like som om, adverbs such as allerede usually come before the finite verb:

• ... som om sommeren allerede er begyndt

So the pattern is:

• main clause: subject + finite verb + adverb
• subordinate clause: subject + adverb + finite verb

This is one of the most important word-order patterns in Danish.

Sommeren is the definite form of sommer, meaning the summer.

Danish adds the definite article to the end of the noun:

• sommer = summer
• sommeren = the summer

In this sentence, Danish uses the definite form because it is talking about summer as the season that now seems to have started. English often says simply summer, but Danish frequently uses the definite form in cases like this.

So:

• sommeren allerede er begyndt
  literally = the summer has already begun

even though natural English is usually just summer has already begun.

Yes. Er begyndt is a perfect-style form meaning has begun / is begun, depending on how literally you analyze it. In normal English translation here, it means has already begun.

With verbs like begynde, Danish often uses være in expressions where something has started:

• Filmen er begyndt = The film has begun
• Mødet er begyndt = The meeting has begun
• Sommeren er begyndt = Summer has begun

So in this sentence:

• sommeren allerede er begyndt = summer has already begun

For a learner, the safest approach is to learn er begyndt as a very common pattern.

Allerede means already.

It shows that something seems to have happened sooner than expected:

• Sommeren er allerede begyndt = Summer has already begun

In this sentence, it strengthens the feeling that the atmosphere is so summery that it seems the season has started already, even if it may not officially have.

Because it is inside a subordinate clause, it comes before er:

• ... som om sommeren allerede er begyndt

Here når means when in a general or repeated sense.

The sentence is not about one single occasion. It means that whenever / when we eat strawberries on the terrace after a warm day, it feels like summer.

So:

• når vi spiser jordbær... = when we eat strawberries...

Use når for things that happen repeatedly or generally.

By contrast:

• da is often used about one specific past occasion
• mens means while, focusing on two things happening at the same time

So når is the right choice here.

Here jordbær means strawberries, and the singular and plural forms can look the same in the indefinite form.

• et jordbær = a strawberry
• jordbær = strawberries

In this sentence, the meaning is clearly plural because people normally eat strawberries, not just one strawberry, and the English meaning confirms that.

This is common in Danish with some nouns: the plural form does not always add a visible ending.

Because på is the normal preposition for being on a terrace, balcony, island, surface, and similar places.

So:

• på terrassen = on the terrace

Using i would suggest being inside something, which does not fit a terrace.

Also, terrassen is definite:

• terrasse = terrace
• terrassen = the terrace

So på terrassen means on the terrace, usually a specific terrace known from the context, such as the one at the house.

Both forms mean the/a, but Danish has two different article patterns.

1. En varm dag
   This is the indefinite article

   • noun phrase:
   • en dag = a day
   • en varm dag = a warm day

2. Terrassen
   This is the definite suffix attached to the noun:

   • terrasse = terrace
   • terrassen = the terrace

So:

• efter en varm dag = after a warm day
• på terrassen = on the terrace

Danish usually puts definiteness at the end of the noun instead of using a separate word like English the.

It has a main clause followed by two subordinate parts:

• Det føles næsten
  main clause = It almost feels

• som om sommeren allerede er begyndt
  comparison clause = as if summer has already begun

• når vi spiser jordbær på terrassen efter en varm dag
  time clause = when we eat strawberries on the terrace after a warm day

So the whole sentence works like this:

It almost feels
as if summer has already begun
when we eat strawberries on the terrace after a warm day

This is a very natural Danish sentence with a main impression first, then the explanation of that feeling.