Jeg synes historien er fascinerende.

Both are correct. Synes and syns are alternative spellings in Bokmål.

• jeg synes = more traditional/common in many learning materials
• jeg syns = also accepted and very common in everyday writing

So Jeg synes historien er fascinerende and Jeg syns historien er fascinerende mean the same thing.

Here, synes means to think / to find / to feel that when giving a personal opinion.

So Jeg synes historien er fascinerende means something like:

• I think the story is fascinating
• I find the story fascinating

It is used for opinions, not for deep logical reasoning in the same way as English think can sometimes be.

In Norwegian, synes is usually the natural choice for personal opinions about something.

• Jeg synes historien er fascinerende = I think/find the story fascinating
• Jeg tenker more often means I am thinking or my view is in a more reflective or process-oriented way

A learner should usually remember:

• synes = opinion
• tenker = thinking, considering, reflecting

So in this sentence, synes sounds more natural.

Historien is the definite form, meaning the story.

In Norwegian, nouns often add the definite article as an ending instead of using a separate word like the.

For en historie:

• en historie = a story
• historien = the story

So:

• Jeg synes en historie er fascinerende = I think a story is fascinating
• Jeg synes historien er fascinerende = I think the story is fascinating

Usually yes in a sentence like this, but historie can also mean history in some contexts.

So historien can mean:

• the story
• the history

In Jeg synes historien er fascinerende, the exact meaning depends on context. If the learner has already been shown the meaning the story, then that is the intended reading here.

Fascinerende is an adjective ending in -ende, and these adjectives usually keep the same form.

Examples:

• en fascinerende historie = a fascinating story
• historien er fascinerende = the story is fascinating
• fascinerende bøker = fascinating books

Unlike many other adjectives, it does not usually take extra endings like -t or -e in these common uses.

Norwegian often leaves out at (that) after verbs like synes, especially in simple sentences.

So Norwegian says:

• Jeg synes historien er fascinerende

Literally this is closer to:

• I think the story is fascinating

You could also say:

• Jeg synes at historien er fascinerende

This is grammatically correct, but the version without at is very natural and common.

The sentence has a very normal Norwegian main-clause structure:

• Jeg = subject
• synes = verb
• historien er fascinerende = what you think

So it is:

• subject + verb + rest

Inside the second part, historien er fascinerende is also in normal order:

• historien = subject
• er = verb
• fascinerende = adjective/complement

This makes the whole sentence straightforward for learners.

Yes, but it means something different.

• Jeg synes historien er fascinerende = I think the story is fascinating
• Jeg synes om historien = I like the story

So:

• synes + clause = express an opinion
• synes om + noun = like something / be fond of something

This is a very important distinction.

Not in this structure. In Norwegian, you normally need the full clause:

• Jeg synes historien er fascinerende

You are saying what the story is, so er is needed.

You would not normally say:

• Jeg synes historien fascinerende

That sounds incomplete or unnatural in standard everyday Norwegian.

Although the direct meaning is clear, a natural English translation could vary slightly:

• I think the story is fascinating.
• I find the story fascinating.

Both match the Norwegian well. The second one is especially close in feeling to synes in this sentence.

It is neutral and natural. You can use it in both speech and writing.

It is not especially formal, and it is not slangy either. That makes it a very useful model sentence for learners.