Learn German: Der Kuchen bleibt in der Form, bis er abkühlt. - The cake stays in the pan until it cools down.

The word bis (until) is a subordinating conjunction. This means it introduces a dependent clause and forces the conjugated verb to the very end of that clause.

I thought abkühlen was a separable verb. Why isn't it written as kühlt ... ab here?

You are right that abkühlen (to cool down) is a separable verb. Normally in a main clause, the prefix ab goes to the end. However, because bis pushes the conjugated verb to the end of the sentence, the prefix and the verb stem are forced back together into a single word: abkühlt.

Why does the second part use er when the English translation says 'it'?

In German, pronouns must match the grammatical gender of the noun they replace. Since der Kuchen (the cake) is a masculine noun, we have to use the masculine pronoun er to refer back to it, even though we say 'it' in English.

Why does the sentence use in der Form if the word for pan or mold is feminine (die Form)?

The preposition in is a two-way preposition. Because the cake is staying in one place and not moving to a new location, it answers the question 'where?' (wo?). This means we use the dative case, and the feminine article die changes to der.