"What is bigger than the tallest mountain?"
Translation:Was ist größer als der höchste Berg?
As an American that lived most of my life in "The Mountains" I've never heard anyone refer to mountains as "tall." I notice that "Tall" is usually reserved for things whose height is significantly greater than its base or width, thus it calls attention to the vertical dimension. Thus trees, buildings, and people (especially with lanky builds) are normally/always called "tall" and mountains are called "high." If a person or building is also of a wide or thick build we just call it "big" -calling attention to no particular dimension...
To my ears high mountains are much more common, and growing up on Te Waipounamu, the South Island of New Zealand, I've had my fare share of high mountains. But a tall mountain doesn't sound completely wrong, just uncommon. Perhaps if one was being dramatic/poetic and particularly referencing a mountain that you you have interacted with, it might work. "This sure is a tall mountain that we picked to climb"
It is "der höchste Berg."
the adjective "hoch" is not regular in the comparative.
- am höchsten
and it is also unregulare in this dircetion:
- ein hoher Baum. der hohe Baum. Der Baum ist hoch.
- eine hohe Tanne. die hohe Tanne. Die Tanne ist hoch.
- ein hohes Gebäude. das hohe Gebäude. Das Gebäude ist hoch.