Learn Finnish: Minä pidän rohkeudesta.

To form an abstract noun from an adjective ending in -ea or -eä, you replace the final -a or -ä with -us or -ys. So, rohkea becomes rohkeus. This is a common variation of the -uus/-yys suffix you are learning in this lesson.

Why does the stem of rohkeus become rohkeude- in this sentence?

Words ending in -us or -ys that are derived from adjectives have a stem ending in -ute- or -yte-. For rohkeus, the basic stem is rohkeute-. Because we are adding the elative suffix -sta, which closes the syllable, the strong consonant t weakens to d through consonant gradation, resulting in rohkeudesta.

Why is the word for courage in the elative case (-sta) here?

The verb pitää is a rection verb. When it means "to like", it always forces the noun that follows it into the elative case (-sta/-stä). While English speakers "like something", Finnish speakers "like from something".

Could I just say Pidän rohkeudesta without minä?

Yes, absolutely! The -n ending on the verb pidän already tells us the subject is "I", so the pronoun minä is entirely optional. Including it just adds a little extra emphasis to the fact that you are the one who likes it.