Learn Swedish: Trafiken är långsam idag. - The traffic is slow today.

Adjectives in Swedish must match the gender of the noun they describe. Because trafik is an en-word (which becomes trafiken in the definite form), we use the base form långsam. If we were describing an ett-word like tåget (the train), it would be långsamt.

Why use the definite form trafiken instead of just trafik?

In this sentence, you are talking about the specific traffic happening right now in your area, so Swedish uses the definite form (the traffic) just like English does.

How would I add så (so) to this sentence to say "so I am late"?

You can simply add så jag är sen to the end. Because så is a coordinating conjunction, it links two main clauses without changing the word order of the second one. The full sentence would be: Trafiken är långsam idag, så jag är sen.

Is it better to write idag as one word or two (i dag)?

Both are completely correct and mean the exact same thing. Writing it as one word (idag) is extremely common in everyday modern Swedish, while writing it as two words (i dag) is the slightly more traditional spelling recommendation.