Det jag alltid tar med på picknick är en termos, en filt och vindruvor.

This is a very common Swedish pattern. Det jag alltid tar med ... är ... literally means The thing that I always bring ... is ...

Here, det does not mean a specific it. It works more like that which or the thing that.

So the structure is:

• Det = the thing / that which
• jag alltid tar med = that I always bring
• är = is
• en termos, en filt och vindruvor = a thermos, a blanket, and grapes

A more natural English equivalent is often:

• What I always bring on a picnic is a thermos, a blanket, and grapes.

So this is a cleft-like or focusing structure used to highlight the things being brought.

Ta med is a Swedish verb phrase meaning to bring or to take along.

In the sentence, it appears as tar med because:

• ta = infinitive form, to take
• tar = present tense, take / am taking
• med = with, but together ta med means bring / take along

Examples:

• Jag tar med kaffe. = I’m bringing coffee.
• Ska du ta med en filt? = Are you bringing a blanket?

It is split because Swedish often uses verb + particle combinations, similar to English phrasal verbs like take along or bring along.

Both word orders can exist in Swedish, but in this kind of relative clause, jag alltid tar med is very natural.

The part jag alltid tar med is a subordinate/embedded clause after det. In subordinate clauses, Swedish word order is often different from main clauses.

A useful comparison:

• Main clause: Jag tar alltid med en filt.
• Embedded clause: det jag alltid tar med

In main clauses, the finite verb often comes earlier. In subordinate clauses, adverbs like alltid often come before the finite verb:

• att jag alltid tar med ...
• det jag alltid tar med ...

So this word order is normal and grammatical.

På picknick means on a picnic or for a picnic in a general sense.

In Swedish, some activities and situations are often expressed without an article, especially when you mean the activity in general rather than one specific event.

So:

• på picknick = on a picnic / when having a picnic
• på en picknick = on a specific picnic

In your sentence, på picknick sounds natural because it describes the general context: what the speaker always brings when going on a picnic.

Compare:

• Vi är på picknick. = We’re on a picnic.
• Det jag tar med på en picknick i morgon ... = What I’m bringing to a picnic tomorrow ...

Both are possible, but they mean slightly different things.

Because en termos and en filt are singular countable nouns, while vindruvor is plural.

In Swedish:

• singular countable nouns usually need an article in the indefinite form:
  • en termos
  • en filt
• plural indefinite nouns usually do not take an article:
  • vindruvor

So:

• en termos = a thermos
• en filt = a blanket
• vindruvor = grapes

This works similarly to English:

• a thermos
• a blanket
• grapes

You would only add something else if you wanted to specify them, for example:

• några vindruvor = some grapes

But just vindruvor is perfectly natural.

• en termos = common gender
• en filt = common gender
• en vindruva = common gender

So all three use en, not ett.

Their forms are:

• en termos / termosen / termosar
• en filt / filten / filtar
• en vindruva / vindruvan / vindruvor

Notice that vindruvor is the plural of vindruva.

This is a good question. The verb agrees with the subject, not with the list that comes after it.

The subject here is:

• Det jag alltid tar med på picknick

That whole phrase is treated as singular, so Swedish uses är = is.

In other words, the sentence structure is basically:

• What I always bring on a picnic is X, Y, and Z.

English does the same thing:

• What I bring is a thermos, a blanket, and grapes.

Even though multiple items follow, what I bring is grammatically singular.

Yes. Picknick is a standard Swedish noun borrowed from the same source as English picnic.

It is usually:

• en picknick
• definite form: picknicken

But in many common expressions, Swedish often leaves out the article:

• gå på picknick = go on a picnic
• vara på picknick = be on a picnic

So in your sentence, på picknick is a very natural fixed expression.

Yes, you could say Det som jag alltid tar med på picknick är ..., and it is grammatical.

There is a small style difference:

• Det jag alltid tar med ... is very common and slightly more direct.
• Det som jag alltid tar med ... is also common and can feel a little more explicit.

Both mean essentially the same thing here:

• What I always bring on a picnic is ...

Swedish often allows both det jag ... and det som jag ... in this kind of structure.

Yes. Ta med can correspond to both bring and take along, depending on perspective.

For example:

• Jag tar med en filt.
  This could be translated as:
  • I’m bringing a blanket.
  • I’m taking a blanket with me.

English distinguishes bring and take more clearly depending on direction, but Swedish ta med is often broader.

So in this sentence, the natural English translation is usually bring, because the focus is on what someone brings to a picnic.