Learn Swedish: Hon sjunger mycket bra i kyrkan. - She sings very well in the church.

In Swedish, i is used when someone or something is physically inside a space or building. Since she is singing inside the church walls, i is the correct preposition. You will also hear i kyrkan used as a set phrase for attending a service, similar to saying "at church" in English.

Why does kyrkan end with an -n?

In Swedish, the word for "the" isn't a separate word before the noun; it gets attached to the end! The basic word for a church is en kyrka. By adding the -n to the end, it becomes kyrkan, which means "the church".

I thought bra meant "good". Why does it translate to "well" in this sentence?

In Swedish, bra does double duty. It works both when describing a thing ("good") and when describing how an action is done ("well"). Since she is doing the action of singing, bra translates to "well" here.

Doesn't mycket usually mean "much" or "a lot"?

It can mean "a lot", but it also means "very"! When you put mycket directly in front of a describing word like bra, it translates to "very". So mycket bra means "very well".