Learn Swedish: Nörden, han är väldigt framgångsrik idag. - The nerd, he is very successful today.

In this sentence, Nörden is placed completely outside the main clause to create strong emphasis, a structure called left-dislocation. The actual main clause starts with the resumptive pronoun han, which acts as the foundation in the first position. Therefore, the verb är correctly takes the second position right after han.

Why use han here instead of den to refer back to Nörden?

Even though nörd is a standard grammatical en-word (which normally takes den), it refers to a person. In Swedish, when a noun clearly represents a specific human being, you generally use the natural gender pronouns han (he) or hon (she) as the resumptive pronoun instead of the grammatical pronoun den.

Why doesn't framgångsrik get an -t ending when it comes after väldigt?

The word väldigt is an adverb here meaning 'very', so it does not affect the ending of the adjective it modifies. The adjective framgångsrik must agree with the subject han (and by extension, Nörden). Since the subject is a singular en-word, framgångsrik stays in its basic form without an added -t.