Äventyret började plötsligt nära kusten.

Because Swedish usually puts the definite article at the end of the noun.

• ett äventyr = an adventure
• äventyret = the adventure

Äventyr is a neuter noun, so its definite singular ending is -et.
So äventyret is the definite form of ett äventyr.

You learn it together with its article:

• ett äventyr = a/the adventure

In Swedish, nouns are either en-words or ett-words.
Since it is ett äventyr, it becomes äventyret in the definite singular.

A useful pattern is:

• ett
  • noun → indefinite singular
• noun + -et → definite singular

So:

• ett äventyr
• äventyret

Började is the past tense of börja, which means to begin or to start.

Common forms:

• börja = infinitive, to begin
• börjar = present, begins / is beginning
• började = past, began
• börjat = supine, used in perfect tenses

So in this sentence, började tells you that the action happened in the past.

Here plötsligt is being used as an adverb, meaning suddenly.

It comes from the adjective plötslig = sudden.
A very common way to form an adverb in Swedish is to add -t to the adjective:

• snabb → snabbt = quickly
• tydlig → tydligt = clearly
• plötslig → plötsligt = suddenly

So plötsligt describes how the adventure began.

Yes. Plötsligt can also be the neuter singular form of the adjective plötslig.

For example:

• ett plötsligt ljud = a sudden sound

But in Äventyret började plötsligt nära kusten, it is not describing a noun. It is describing the verb började, so here it is an adverb: suddenly.

This is normal Swedish word order.

The basic structure here is:

• Äventyret = subject
• började = finite verb
• plötsligt = adverb
• nära kusten = place expression

So the sentence follows a common pattern:

Subject + verb + adverb + place

Swedish main clauses also follow the V2 rule, which means the finite verb usually comes in the second position. Since the sentence starts with the subject Äventyret, the verb började comes next.

Here nära is a preposition meaning near.

So:

• nära kusten = near the coast

Swedish nära can also be used in other ways, but in this sentence it introduces a place phrase.

Examples:

• nära huset = near the house
• nära staden = near the city

Because the sentence means the coast, not just coast.

• en kust = a coast
• kusten = the coast

Kust is an en-word, so its definite singular form ends in -en.

That gives:

• en kust
• kusten

So nära kusten means near the coast.

Because they belong to different noun genders in Swedish.

• ett äventyr → äventyret
• en kust → kusten

So:

• ett-word nouns usually take -et in the definite singular
• en-word nouns usually take -en in the definite singular

This is one of the most important noun patterns in Swedish.

Yes, but the meaning shifts slightly.

• nära kusten = near the coast
• vid kusten = by the coast / at the coast

Nära emphasizes closeness.
Vid often means by, at, or next to.

In many contexts both can work, but nära kusten is a bit more directly near the coast.

A rough pronunciation guide for an English speaker could be:

EH-ven-tew-ret BUR-yah-deh PLUTS-ligt NEH-rah KUS-ten

A few notes:

• ä sounds somewhat like the e in bed, though not exactly the same.
• ö in började has no exact English equivalent; it is a rounded front vowel.
• sj-type sounds are not in this sentence, so pronunciation is a bit easier than in some other Swedish sentences.
• The g in plötsligt is not strongly pronounced in normal speech.

If you want a more natural rhythm, try saying it in three chunks:

Äventyret / började plötsligt / nära kusten

It can match either, depending on context.

• börja often corresponds to begin
• but it can also correspond to start

So this sentence could naturally be understood as either:

• The adventure began suddenly near the coast
• The adventure started suddenly near the coast

In many situations, the difference is not important.

Yes. You can say:

Plötsligt började äventyret nära kusten.

That is also correct Swedish.
Because of the V2 rule, the finite verb började still has to come second.

So compare:

• Äventyret började plötsligt nära kusten.
• Plötsligt började äventyret nära kusten.

Both are grammatical, but the second one puts more emphasis on suddenly.