Adverbs are generally made by putting an ending -t to an adjective. If the adjective ends with a d, then the d is changed to a t, except when the vowel has a shorter length, then it becomes -tt. Historically, it would have become -dt no matter what, this is still the case in Danish. If the adjective ends with a t, one adds a new t and shorten the length of the vowel. If the ending is -n, then the n is changed into a t.
Note however, that also adjectives acquires the same ending if the noun is neuter instead of uter. The comparative and superlative forms are the same as for an adjective.
Note that därför is not a formal word in Swedish, and would probably be translated as "that's why" in the sentence above.