Learn Finnish: Pieni kissa ei ole sukassa. - The small cat is not in the sock.

This is consonant gradation! Adding the "in" ending -ssa closes the syllable, which forces the word into its "weak" grade. The double 'k' in sukka simplifies to a single 'k' (suka-), and then we add the ending to get sukassa.

Since double letters are simplifying in this lesson, why does kissa keep its double 's'?

Two reasons! First, consonant gradation only affects the letters P, T, and K; the double 's' in kissa will never simplify. Second, kissa is the subject of the sentence and is in its basic dictionary form here, so no endings have been added to trigger a change anyway.

Why is the verb ole instead of on?

When saying "is not" in Finnish, the word "not" acts like a verb (ei for "it does not"). The main verb then drops its person ending and uses a special base form. For the verb "to be", that negative base form is ole. So, while "is" is on, "is not" is ei ole.

Why do we use the ending -ssa on sukassa instead of -ssä?

Because of vowel harmony! The root word sukka contains the back vowels 'u' and 'a', so its endings must also use back vowels. That gives us -ssa. If the word had front vowels (like y, ä, or ö), we would use -ssä instead.