Learn Swedish: Journalisten undrar om källan är hemlig. - The journalist wonders if the source is secret.

It means 'if' in the sense of 'whether' to introduce a subordinate clause (an indirect question). Swedish uses om for both conditional 'if' (like 'if it rains') and 'whether' (like 'wonders whether').

Since the journalist is wondering a question, why is the word order källan är instead of är källan?

Because the second part of the sentence is an indirect question, which acts as a subordinate clause (bisats). In Swedish subordinate clauses, the subject always comes right before the verb, so the word order stays straight as källan är.

How do we form the word källan from its base form?

The base word is en källa (a source). To make an en-word that ends in -a definite ('the source'), you just add an -n to the end, making it källan.

Why doesn't the adjective hemlig have a -t at the end?

Adjectives must match the noun they describe. Because källan is a singular en-word, the adjective takes its basic form, hemlig. It would only get a -t if it described an ett-word.