Det var mostern som tog med hallon i går.

This is a very common Swedish cleft sentence pattern. It is used to put special focus on one part of the sentence.

• Det var mostern som tog med hallon i går.
• Literally, it is something like: It was the aunt who brought raspberries yesterday.

Here, the speaker is emphasizing mostern — in other words, it was the aunt (not someone else).

This pattern is extremely common in Swedish:

• Det var Johan som ringde. = It was Johan who called.
• Det var i går som det hände. = It was yesterday that it happened.

So det var ... som ... is not strange or unnecessary — it is a normal way to highlight information.

Moster means maternal aunt — your mother’s sister.

Mostern is the definite form, meaning the maternal aunt.

Swedish often adds the definite ending directly to the noun:

• en moster = an aunt
• mostern = the aunt

So mostern is not a separate word like English the + aunt. The -n ending carries the meaning of the.

Not exactly. Swedish is more specific than English here.

• moster = mother’s sister
• faster = father’s sister

English uses aunt for both, but Swedish distinguishes them.

So mostern specifically means the mother’s sister.

Because Swedish usually marks definiteness by adding an ending to the noun.

So instead of saying the aunt, Swedish says:

• mostern

This is called the definite suffix.

Compare:

• en bok = a book
• boken = the book

In this sentence, mostern already includes the, so no extra article is needed.

In this sentence, som means who/that and introduces a relative clause.

• mostern som tog med hallon i går
• the aunt who brought raspberries yesterday

In the cleft structure Det var X som ..., som links the focused element to the rest of the statement.

So:

• Det var mostern som tog med hallon i går = It was the aunt who brought raspberries yesterday

Because ta med is a very common Swedish verb phrase meaning bring or take along.

• ta = take
• ta med = take with / bring

In the past tense:

• tog = took
• tog med = brought / took along

So:

• tog med hallon = brought raspberries

This is a phrasal-style verb, and Swedish has many of them.

Examples:

• Jag tar med kaffe. = I’ll bring coffee.
• Hon tog med sin vän. = She brought her friend.

Yes, particles like med can sometimes move depending on the sentence structure.

Here, tog med stays together because nothing splits it up:

• mostern som tog med hallon i går

But in other sentences, the object can come between the verb and the particle:

• Hon tog med hallon.
• Hon tog hallon med sig. (different structure)
• Hon tog med sig hallon.

With many Swedish particle verbs, word order can vary depending on the type of object and the construction. For a learner, it is best to first remember ta med = bring/take along as a unit.

This is a very common learner question. In Swedish, some nouns have the same form in singular and plural, and hallon is one of them.

• ett hallon = a raspberry
• hallon = raspberries

So the plural does not get a special ending here.

Examples:

• Jag åt ett hallon. = I ate a raspberry.
• Jag åt hallon. = I ate raspberries.

This is normal Swedish.

Because Swedish often leaves out the article when talking about an indefinite quantity or things in a general sense.

• tog med hallon = brought raspberries

This does not mean one specific, already identified set of raspberries in the same way that the raspberries would.

If you wanted the raspberries, Swedish would usually say:

• tog med hallonen

Compare:

• Hon köpte hallon. = She bought raspberries.
• Hon köpte hallonen. = She bought the raspberries.

So hallon here is an indefinite plural/mass-like use.

Because in Swedish, yesterday is written as two words:

• i går

Literally, it originally comes from something like on the previous day, but as a learner you should simply memorize it as the normal expression for yesterday.

Related time expressions:

• i dag = today
• i morgon = tomorrow
• i går = yesterday

That is a very natural position for a time expression in Swedish.

The clause is:

• som tog med hallon i går

Swedish often places time expressions like i går, i dag, på måndag, etc. toward the end of the clause.

You could sometimes move it for emphasis, but the version here is the most neutral and natural.

Both are past tense forms.

• är → var = is → was
• tar → tog = take(s) → took

So the sentence is talking about a completed event in the past.

Present-tense version:

• Det är mostern som tar med hallon i dag.
• It is the aunt who is bringing raspberries today.

In your sentence, both the focusing structure and the action are placed in the past:

• Det var ... som tog ... i går

Yes. A simpler version would be:

• Mostern tog med hallon i går.

That simply means The aunt brought raspberries yesterday.

The difference is emphasis:

• Mostern tog med hallon i går. = neutral statement
• Det var mostern som tog med hallon i går. = emphasizes that it was the aunt

So the original sentence is especially useful when correcting someone, contrasting people, or highlighting who did it.

No, it is quite normal for a relative clause.

Inside the clause, the order is basically:

• som
  • verb + object + time

So:

• som = who/that
• tog med = brought
• hallon = raspberries
• i går = yesterday

This is a standard Swedish relative clause pattern.

A rough pronunciation guide for an English speaker could be:

• Det var mostern som tog med hallon i går
• roughly: deh varr MOSS-tern som toog med HAH-lon ee gor

A few notes:

• det is often pronounced more like de
• mostern has stress on the first syllable
• tog has a long vowel
• går has a vowel sound that does not exist exactly in English, so an approximate gor is only a rough guide

If you are learning pronunciation, it is best to listen to native audio, because Swedish vowel length and melody matter a lot.

Yes, though naturally it depends on context. Swedish speakers use moster and faster normally when family relations matter.

But in everyday speech, people may also simply use the person’s name instead of the relationship term, just as in English.

Still, grammatically and culturally, moster is a completely ordinary Swedish family word.