Learn Finnish: Onpa erikoinen skandaali, kieltämättä. - It sure is a peculiar scandal, undeniably.

The -pa clitic adds a tone of surprise, emphasis, or realization, translating well to 'sure is' in English. It must attach to the very first word of the clause, which is why the verb On is pulled to the front of the sentence.

Why is there no word for 'it' (se) in this sentence?

English requires a dummy subject like 'it' (as in 'it is a scandal'), but Finnish often leaves this out in descriptive sentences. Because the verb On was moved to the front to take the -pa clitic, adding se somewhere else would just sound unnatural here.

Where does the word kieltämättä (undeniably) come from?

It is built from the verb kieltää, which means to deny or forbid. The grammar ending -mättä means 'without doing'. So, kieltämättä literally means 'without denying', which we use exactly like 'undeniably' or 'admittedly' in English.

Why is erikoinen skandaali in the basic nominative case instead of the partitive?

In 'A is B' type sentences, the noun dictates the case. Because a scandal (skandaali) is a distinct, countable thing, both the noun and its adjective (erikoinen) stay in the basic nominative case. If you were describing an uncountable mass noun instead, you would use the partitive case.