Learn Swedish: Äpplet är kallt.

Because the adjective is describing an ett-word (ett äpple). When an adjective describes a singular ett-word, we usually add a -t to the end of the base form (kall becomes kallt).

How do we get the word äpplet (the apple) from ett äpple (an apple)?

To say "the" in Swedish, we add a suffix to the noun instead of using a separate word. For ett-words that already end in a vowel, like äpple, we just add a -t to the end, making it äpplet.

In this lesson, we also learned "Det är kallt" (It is cold). Is kallt following the same rule there?

Yes! When talking about the weather or a general situation ("It is cold"), Swedish uses the dummy subject det, which is technically an ett-word. Because det acts as an ett-word, the adjective takes the exact same -t ending we see here with äpplet.

Is the k in kallt pronounced like a hard "k" or a soft "sh" sound?

It is a hard "k" sound (like in the English word "cold"). In Swedish, k stays hard before "hard vowels" like the a in kallt, as well as o, u, and å.