Learn Finnish: Onko tuo kallio jyrkkä?

You would use the superlative form jyrkin. To form it from jyrkkä, two things happen to the stem. First, consonant gradation changes the double kk to a single k. Second, the final ä drops away to make room for the superlative suffix -in. The sentence would be: Onko tuo kallio jyrkin?

Why is the question particle -ko attached to on instead of another word?

The particle -ko (or -kö) attaches to the specific word you are asking about, which then moves to the front of the sentence. Here, you are asking a standard yes/no question about the state of being, so it attaches to the verb on. If you wanted to ask specifically if THAT cliff is steep (as opposed to another one), you would say Tuoko kallio on jyrkkä?

Does kallio always translate to cliff?

Not necessarily. Kallio refers to a large formation of solid rock or bedrock. Depending on the landscape, it can mean rock, bedrock, or cliff. A sheer cliff face is often called a jyrkänne, which comes from the exact same root as jyrkkä (steep)!

Why does jyrkkä end with ä instead of a?