Hon undrar vad den starka känslan i hjärtat betyder.

Because this is an indirect question, not a direct question.

• A direct question in Swedish takes a question mark and usually has question word order:
  Vad betyder den starka känslan i hjärtat? – What does the strong feeling in (the) heart mean?
• An indirect question is embedded inside another clause (here: Hon undrar … – She wonders …), and the whole sentence is a statement, so it ends with a period:
  • Hon undrar vad den starka känslan i hjärtat betyder.

In indirect questions you do not use question mark intonation or punctuation in Swedish.

In Swedish:

• In a direct question, the verb comes right after the question word:
  • Vad betyder den starka känslan i hjärtat?
• In an indirect question (after verbs like undrar, vet, frågar sig, tänker på), the clause behaves like a subordinate clause:
  • Question word (vad)
  • then the subject (den starka känslan i hjärtat)
  • then the verb (betyder)

So the pattern is:

Hon undrar [vad + subject + verb …]
Hon undrar vad den starka känslan i hjärtat betyder.

Yes, it’s the same question word vad (“what”), but it’s used to introduce an indirect question clause.

• In Vad heter du?, vad is a direct question word.
• In Hon undrar vad den starka känslan i hjärtat betyder, vad works like English “what” in:
  • She wonders what the strong feeling in her heart means.

So it:

• Refers to the meaning of the feeling.
• Introduces a subordinate clause (an embedded question) rather than a stand‑alone question.

Yes, this is the famous “double definiteness” rule in Swedish, and it’s normal and required in this structure.

For a definite noun with an adjective, Swedish usually needs:

den / det / de + adjective + definite noun

Examples:

• den nya bilen – the new car
  (bil → bilen)
• den starka känslan – the strong feeling
  (känsla → känslan)

You cannot say:

• ✗ starka känslan, ✗ den stark känsla here.

You must have:

• a definite article (den)
• plus the definite ending on the noun (-n in känslan)
  when there’s an adjective in front.

So den starka känslan is grammatically the normal way to say “the strong feeling”.

Because känsla is an en-word (common gender), not an ett-word.

• en känsla – a feeling
• känslan – the feeling
• With an adjective in the definite form: den starka känslan

For ett-words you would use det instead:

• ett hjärta – a heart
• hjärtat – the heart
• det starka hjärtat – the strong heart

So:

• den goes with en-words (common gender)
• det goes with ett-words (neuter)

Adjective endings in Swedish change with definiteness and number.

• Indefinite singular en-word: en stark känsla – a strong feeling
• Indefinite singular ett-word: ett starkt hjärta – a strong heart
• Definite singular (any gender) with article den/det/de:
  den starka känslan, det starka hjärtat
• Plural (definite or indefinite): starka känslor – strong feelings

So when the noun is definite and has den/det in front, the adjective takes the -a ending:

den starka känslan (not ✗ den stark känsla)

No, that would be ungrammatical in standard Swedish.

When you have:

• an adjective before a definite noun, you must use the double definiteness structure:

den starka känslan
det stora huset
de långa dagarna

Without den, Swedish reads starka känslan as incorrect; the language expects either:

• en stark känsla (indefinite – a strong feeling)
  or
• den starka känslan (definite – the strong feeling)

Because here “the heart” is meant, not just “a heart”.

• ett hjärta – a heart (indefinite)
• hjärtat – the heart (definite)

The -t is the definite ending for this ett-word:

• ett hjärta → hjärtat
• ett hus → huset
• ett brev → brevet

In the phrase i hjärtat, Swedish usually means “in (the) heart”, and in context that’s naturally understood as her own heart.

The preposition i means “in / inside”, which fits the metaphorical idea of an internal feeling.

• i hjärtat – in the heart
  (inside, emotionally or physically)
• på hjärtat – literally on the heart (would sound odd here; might be used physically, like something lying on top of a heart in a medical context, but that’s very different.)

For feelings and emotions located “in one’s heart”, Swedish uses i:

• en värme i hjärtat – a warmth in (one’s) heart
• en oro i hjärtat – an anxiety in (one’s) heart

Swedish often uses a bare definite noun for body parts and similar things when it’s clear they belong to the subject.

• Hon har ont i huvudet. – Her head hurts / She has a headache.
• Han tvättar händerna. – He washes his hands.

The possessor is understood from context (the subject). So:

• Hon undrar vad den starka känslan i hjärtat betyder.
  is naturally interpreted as
  She wonders what the strong feeling in her heart means.

If you say:

• i sitt hjärta – emphasizes that it is her own heart (reflexive, tied to the subject).
• i hennes hjärta – often sounds more contrastive, like “in her heart (not someone else’s)”, or can refer to another female’s heart depending on context.

So i hjärtat is the most neutral and idiomatic here.

Betyda and mena correspond to two different uses of “mean” in English.

• betyda – what a word, symbol, event, or feeling signifies:

  • Vad betyder det här ordet? – What does this word mean?
  • Vad betyder den starka känslan i hjärtat? – What does the strong feeling in (her) heart mean? / What is its significance?

• mena – what a person intends to say or communicate:

  • Vad menar du? – What do you mean? (What are you trying to say?)

A feeling doesn’t “intend” anything in a linguistic sense; it has a significance. So:

• betyder is the correct and natural choice here.
• ✗ Vad menar den starka känslan i hjärtat? would sound wrong or at least very odd.

They are not interchangeable:

• Hon undrar – She wonders / She is wondering

  • This is about her inner thoughts or curiosity.
  • She might not be speaking to anyone.

• Hon frågar – She asks

  • This normally implies she is asking someone a question.

So:

• Hon undrar vad den starka känslan i hjärtat betyder.
  – She is wondering to herself about the meaning.

If you want “She asks what the strong feeling in her heart means”, you’d usually specify whom she asks:

• Hon frågar läkaren vad den starka känslan i hjärtat betyder.
  – She asks the doctor what the strong feeling in her heart means.

The present tense betyder is normal because Swedish often uses the present for:

• current, general, or timeless meanings
• dictionary-like meaning or ongoing significance

Here, the feeling’s meaning is treated as something that generally/now has a certain significance, so:

• Hon undrar vad den starka känslan i hjärtat betyder.
  – She wonders what it means (now / in general).

You could say betydde in a specific past context:

• Igår kände hon en stark känsla i hjärtat och undrade vad den betydde.
  – Yesterday she felt a strong feeling in her heart and wondered what it meant.

But in the original, present tense betyder is the natural default.