Leden, utsikten och solnedgången var fantastiska.

Because they are in the definite singular form: the trail, the view, the sunset.

In Swedish, definiteness is often shown by adding an ending to the noun instead of using a separate word like the.

• en led → leden
• en utsikt → utsikten
• en solnedgång → solnedgången

So the -en here is basically doing the job that the does in English.

Because in Swedish, when a noun is definite and stands on its own, the definite ending is usually enough.

So Swedish says:

• leden = the trail
• utsikten = the view
• solnedgången = the sunset

A separate definite word like den/det/de is typically used when there is also an adjective before the noun:

• den fantastiska utsikten = the fantastic view

But in your sentence, there is no adjective directly before each noun, so the noun ending alone marks definiteness.

Because the past tense of vara (to be) is var for both singular and plural in standard Swedish.

So:

• Den var fantastisk. = It was fantastic.
• De var fantastiska. = They were fantastic.

Unlike English, Swedish does not change the past tense form of vara from was to were. It stays var.

Because the subject is plural overall.

Even though each noun is singular by itself, the full subject is a list of three things:

• Leden
• utsikten
• och solnedgången

Together, that means they, so the adjective must be in its plural form.

For adjectives like fantastisk, the forms are:

• fantastisk — common gender singular
• fantastiskt — neuter singular
• fantastiska — plural

So:

• Utsikten var fantastisk. = The view was fantastic.
• Huset var fantastiskt. = The house was fantastic.
• Leden, utsikten och solnedgången var fantastiska. = The trail, the view, and the sunset were fantastic.

Because agreement follows the whole subject, not each item separately.

A list joined by och (and) is treated as plural:

• Katten och hunden är söta. = The cat and the dog are cute.
• Leden, utsikten och solnedgången var fantastiska.

So even though leden, utsikten, and solnedgången are each singular nouns, the combined subject is plural.

Because Swedish uses compound nouns very freely, much more than English.

solnedgång is made from:

• sol = sun
• nedgång = going down / descent

Together:

• solnedgång = sunset
• solnedgången = the sunset

This is very normal in Swedish. English often writes similar ideas as two words or as a fixed single word, but Swedish strongly prefers building one compound noun.

Other examples:

• sommarhus = summer house
• sjukhus = hospital
• fotbollsmatch = football match

It follows a very common Swedish pattern:

Subject + verb + adjective/complement

So here:

• Leden, utsikten och solnedgången = subject
• var = verb
• fantastiska = subject complement / predicate adjective

This is directly parallel to English:

• The trail, the view, and the sunset were fantastic.

So the word order is very straightforward.

Because Swedish usually does not use a serial comma (Oxford comma) before och in a simple list.

So this is standard:

• Leden, utsikten och solnedgången var fantastiska.

This is similar to British English style, and also common in many forms of English. A comma before och is usually unnecessary unless it helps avoid confusion.

No, that would sound wrong in standard Swedish.

Since the subject is plural, the predicate adjective should also be plural:

• ... var fantastiska ✅
• ... var fantastisk ❌

If you wanted the singular form fantastisk, you would need a singular subject, for example:

• Utsikten var fantastisk.

A natural pronunciation would be roughly:

LEH-den, OOT-sik-ten ok SOOL-ned-gong-en var fan-TAS-tis-ka

A few helpful notes:

• led in leden has a long e sound.
• utsikten has stress on the first syllable: UT-sikten
• solnedgången has main stress at the start: SOL-nedgången
• gång has the å sound, roughly like the vowel in British law
• fantastiska is stressed on -tas-

Pronunciation varies a bit by region, but the main stress pattern above is a good guide.

Not in normal Swedish if you mean specific things that were experienced or seen.

Without the definite endings, the sentence would sound unnatural here. Swedish usually wants the definite form when referring to specific, identifiable things:

• leden, utsikten och solnedgången = the trail, the view, and the sunset

The indefinite forms would be:

• en led
• en utsikt
• en solnedgång

But a bare indefinite list like led, utsikt och solnedgång would not normally fit this sentence.