Learn Swedish: Rytmen är svår att återskapa. - The rhythm is hard to recreate.

You are thinking of sentences starting with det, like 'Det är svårt att återskapa rytmen' (It is hard to recreate the rhythm). In that case, svårt describes the neuter dummy pronoun det. But in this sentence, the object (rytmen) has been moved to the front to become the subject. Since rytm is an en-word, the adjective must agree with it, so we use svår.

If I want to use svårt instead, how do I rewrite the sentence?

You would use the dummy pronoun det as the subject: Det är svårt att återskapa rytmen. This means exactly the same thing, but keeps the adjective in the neuter form because it grammatically refers back to det.

How does the prefix åter- work in återskapa?

The prefix åter- means 'again' or 'back', very much like the English prefix 're-'. Combined with skapa (to create), you get återskapa (to recreate). You will also see it in words like återvända (to return) or återuppta (to resume).

Is there a reliable way to know that rytm is an en-word?