Learn Swedish: Han viskade att det bara var ett skämt. - He whispered that it was just a joke.

Because the main reporting verb is in the past tense (viskade). When reporting what someone said in the past, Swedish "backshifts" the tense in the subordinate clause to match it. He originally whispered, "Det är bara ett skämt" (It is just a joke), but in indirect speech, är shifts back to var.

Why is the word order det bara var? Shouldn't the verb come before bara?

This is because of the word order rule for subordinate clauses. In a dependent clause starting with att (that), sentence adverbs like bara (just/only) or inte (not) must be placed before the verb. If you said this as an independent main sentence, the verb would come first: "Det var bara ett skämt."

What is the verb form of ett skämt?

The verb is att skämta (to joke). A very common phrase you will hear in Swedish is "Jag skämtar bara!" (I am just joking!). Another equally common pair of words you can use for this is ett skoj (a joke) and att skoja (to joke).

Why is skämt pronounced with a soft "sh"-like sound instead of a hard "sk"?