Learn Finnish: Mikä on repussa? - What is in the backpack?

This is consonant gradation in action! Adding the -ssa ("in") ending closes the final syllable, which forces the strong double consonant pp to weaken into a single p. That is why reppu becomes repussa.

How do we know whether to use -ssa or -ssä here?

We use -ssa because of vowel harmony. The word reppu contains the back vowel u. (The vowel e is neutral, so the u takes charge). Since it acts as a back-vowel word, it requires the back-vowel ending -ssa.

Why use mikä instead of mitä for "what"?

Mikä is the basic, nominative form used to identify a complete, countable subject. You are asking "What (specific thing) exists in the backpack?". Mitä is the partitive form, which you would use if asking about an ongoing action (like "What are you doing?") or an uncountable substance.

Is there a word for "the" in this sentence?

No, Finnish does not have articles like "a" or "the". Repussa translates directly to both "in a backpack" and "in the backpack". Context tells us which one makes the most sense in English!