Learn Finnish: Tiedän, että opit ääntämään hyvin kuuntelemalla. - I know that you learn to pronounce well by listening.

Kuuntelemalla is the third infinitive adessive form of the verb kuunnella (to listen). You make it by taking the strong stem (kuuntele-), adding the third infinitive marker -ma-, and then the adessive ending -lla. It translates directly to "by listening" and tells us how or by what means the learning happens.

Why is the word for "to pronounce" written as ääntämään instead of its basic dictionary form ääntää?

This is because of the verb oppia (to learn). In Finnish, oppia requires the verb that follows it to be in the third infinitive illative form, which ends in -maan or -mään. So "learn to pronounce" must always be oppia ääntämään.

I noticed the basic verb kuunnella (to listen) has two Ns, but kuuntelemalla has an N and a T. Why?

That is consonant gradation in action! Verb type 3 words like kuunnella are actually in their "weak" grade in the dictionary form (with -nn-). To make the third infinitive forms (like the -malla ending), you must use the strong stem. The strong grade of -nn- is -nt-, which gives us the stem kuuntele-.

Is the comma before että (that) required?