Остров казался раем.

Because the verb казаться normally takes a noun in the pattern казаться кем? чем? — literally to seem to be someone/something.

So:

• остров = island, nominative, the subject
• раем = paradise, instrumental, the thing it seemed to be

This is very common in Russian:

• Он казался врачом. = He seemed to be a doctor.
• Эта идея кажется странной. = This idea seems strange.
  (With adjectives, you usually use the short/long adjective form instead of a noun.)

So in Остров казался раем, the instrumental marks the role or identity the island seemed to have.

Because казался agrees with остров, which is:

• singular
• masculine

In the past tense, Russian verbs agree with the subject in gender and number:

• казался — masculine singular
• казалась — feminine singular
• казалось — neuter singular
• казались — plural

Examples:

• Деревня казалась тихой. = The village seemed quiet.
• Море казалось спокойным. = The sea seemed calm.
• Горы казались далёкими. = The mountains seemed far away.

Here, -ся is part of the verb казаться, which means to seem or to appear.

It is related historically to reflexive forms, but in modern Russian you should learn казаться as a whole dictionary verb. It does not literally mean to show oneself in normal usage.

Compare:

• казаться = to seem
• показаться = to seem, to appear for a moment / to come into view

So in this sentence, just treat казался as the past tense of казаться.

No. After казаться, a noun normally goes in the instrumental case, so:

• correct: Остров казался раем
• incorrect: Остров казался рай

This is one of those verb + case patterns that just has to be learned.

Normally, no. Казаться does not usually use как before a noun in this structure.

Natural Russian is:

• Остров казался раем.

If you use как, it sounds unnatural here. Russian prefers the instrumental after казаться.

Compare:

• Остров был как рай. = The island was like paradise.
• Остров казался раем. = The island seemed like paradise / seemed to be paradise.

Both can be translated similarly in English, but the Russian structures are different.

The difference is appearance vs reality.

• Остров был раем. = The island was paradise.
  This sounds like a direct statement of fact or strong judgment.

• Остров казался раем. = The island seemed like paradise.
  This emphasizes impression, appearance, or someone’s perception.

So казался is less definite and more subjective.

No, Russian word order is fairly flexible. Остров казался раем is the most neutral order.

You could also say:

• Раем казался остров.

This changes the emphasis. It highlights раем more strongly, something like:

• Paradise — that’s what the island seemed.

So the basic meaning stays the same, but the focus changes.

Because Russian has no articles.

So остров can mean:

• an island
• the island

and раем can mean:

• paradise
• a paradise, depending on context

English has to choose a/the, but Russian leaves that to the situation.

Yes. The present-tense version is:

• Остров кажется раем. = The island seems like paradise.

Other forms:

• Остров казался раем. = The island seemed like paradise.
• Остров будет казаться раем. = The island will seem like paradise.

So казался is simply the past-tense form.

Usually yes, though it can also mean heaven depending on context.

In this sentence, раем most naturally means paradise, because it is being used figuratively:

• Остров казался раем. = The island seemed like paradise.

That sounds much more natural in English than The island seemed like heaven, though that translation is possible in some contexts.