"No one is as scary as him."

Translation: Nikt nie jest taki straszny jak on.

[Miknairb]

I don't understand why both 'tak' and 'taki' are correct here. How do you know when to use tak and when to use taki?

[Jellei]

"Taki" works for adjectives (well, masculine ones, as it also changes with gender).

"Tak" works with adverbs (its first usage) and adjectives as well.