Learn Swedish: Han är jonglör, men han är helt oförberedd. - He is a juggler, but he is completely unprepared.

In Swedish, when you state someone's profession, nationality, or general role using the verb vara (to be) or bli (to become), you usually leave out the article. You only use it if you add a descriptive adjective, such as: han är en skicklig jonglör (he is a skilled juggler).

Since this lesson is about 'inte ens' (not even), how would I change the second clause to say 'but he isn't even prepared'?

You would replace helt (completely) with the focus particle inte ens and remove the negative prefix o- from the adjective. The clause becomes: men han är inte ens förberedd. Because inte ens acts as a single negative unit, it naturally takes the typical spot of the sentence adverb right after the verb är in this main clause.

What if I wanted to emphasize the subject instead, saying 'Not even HE is completely unprepared'?

You would place inte ens directly before the specific constituent it modifies. To focus on the subject, you put it right before 'han': men inte ens han är helt oförberedd. This shows how inte ens can jump around the sentence depending on exactly what word or phrase you want to emphasize.

Why does the adjective 'oförberedd' end with a double 'd' here?