Vandringen på ön var brant.

Vandringen is the definite singular form of vandring, which is an en-word.

• vandring = hike / walk / trek / wandering, depending on context
• vandringen = the hike / walk / trek

So the ending -en works like the English definite article the.

In Swedish, the definite article is often attached to the noun as an ending.

So instead of saying:

• the hike
• the island

Swedish says:

• vandringen
• ön

Here:

• vandring
  • -en = vandringen
• ö
  • -n = ön

This is very normal in Swedish.

Ö means island in its basic, indefinite form.

• ö = an island
• ön = the island

Because the noun ends in a vowel, the definite ending is just -n, not -en.

So:

• en ö = an island
• ön = the island

This pattern is common with nouns ending in vowels.

Swedish usually uses på for being on an island.

So:

• på ön = on the island

This is one of those preposition patterns you largely have to learn as a chunk. For islands, på is very common and often the natural choice.

A learner may want to translate directly from English and think on must always match på and in must always match i, but in Swedish prepositions do not always line up perfectly with English. In this case, though, på ön does match English on the island quite well.

Because på ön is describing which hike we are talking about: the hike on the island.

So Vandringen på ön is one noun phrase:

• vandringen = the hike
• på ön = on the island

Together: the hike on the island

If you move it, the emphasis changes. For example:

• Vandringen på ön var brant. = The hike on the island was steep.
• På ön var vandringen brant. = On the island, the hike was steep.

The second version is possible, but it emphasizes the location more.

Var is the past tense of är, which comes from the verb vara = to be.

So:

• är = is / are
• var = was / were

In this sentence, var simply means was.

Here brant is a predicative adjective, meaning it comes after var and describes the subject.

In Swedish, predicative adjectives usually agree with the noun in number and sometimes gender. But brant is a slightly tricky adjective because its common singular and neuter singular forms look the same:

• en brant stig = a steep path
• ett brant berg = a steep mountain
• stigarna är branta = the paths are steep

So in the sentence Vandringen på ön var brant, brant is the correct singular form.

You would use branta for plural or definite plural contexts, for example:

• Vandringarna var branta = The hikes were steep.

Yes, it can be natural, especially if you mean the hike or route involved steep terrain.

However, in some contexts Swedish speakers might more naturally describe:

• stigen = the path
• backen = the slope
• terrängen = the terrain

For example:

• Stigen på ön var brant. = The path on the island was steep.
• Terrängen på ön var brant. = The terrain on the island was steep.

But Vandringen på ön var brant is still understandable and can be perfectly fine if you mean the hike itself was steep.

Not if you just want to describe it as steep.

• var brant = was steep
• blev brant = became steep / got steep

So blev suggests a change, as if the hike started out not steep and then turned steep.

In your sentence, var is the normal choice because it simply describes the hike.

The indefinite version would be:

En vandring på en ö var brant.

That means something like A hike on an island was steep.

Compare:

• Vandringen på ön var brant. = The hike on the island was steep.
• En vandring på en ö var brant. = A hike on an island was steep.

The original sentence is definite, so it refers to a specific hike and a specific island.

Yes. This is very standard.

The structure is:

• Vandringen på ön = subject
• var = verb
• brant = subject complement / adjective

So the sentence follows a simple pattern:

Subject + verb + complement

That makes it a very typical Swedish statement sentence.