Learn Swedish: Knappt hinner astronomen observera förmörkelsen. - The astronomer barely has time to observe the eclipse.

This is an example of stylistic fronting. By moving a restrictive adverb like knappt (barely) to the first position, it creates rhetorical emphasis. It makes the situation sound more dramatic, emphasizing just how little time the astronomer actually has.

Since Knappt is at the front, why does the verb hinner come before the subject?

This is the classic Swedish V2 (verb-second) rule. Because the adverb knappt takes up the first position in the sentence, the conjugated verb hinner must take the second position. This forces the subject astronomen into the third position.

Could I say har tid att instead of hinner here?

You could, but hinner is much more natural here. While har tid just means possessing the free time to do something, hinner implies successfully managing to do something within a limited time window. It fits the urgent tone of a fleeting eclipse perfectly.

How is the word förmörkelsen (the eclipse) formed?

It is built from the adjective mörk (dark). Adding the prefix för- and suffix -else creates the noun förmörkelse, which literally means a darkening. The final -n is the standard definite suffix making it the eclipse.