Learn German: Kletterst du auf den Berg? Es wäre sehr mutig. - Are you climbing up the mountain? It would be very brave.

auf is a two-way preposition. Because you are moving towards a destination (climbing up onto the mountain), you use the accusative case (den). If you were already on the mountain and just climbing around up there, you would use the dative case (dem).

What verb form is wäre?

wäre is the conditional form of sein (to be), and it translates to 'would be'. You use it to talk about hypothetical or imagined situations, like imagining how brave the climb might be.

If I wanted to state 'Nevertheless, you are climbing up the mountain', how would I use trotzdem at the beginning of the sentence?

You would say Trotzdem kletterst du auf den Berg. Since trotzdem takes up the first position in the sentence, the verb kletterst must stay in the strict second position. This pushes the subject du right after the verb.

Could I use Das instead of Es in the second sentence?

Yes! You can absolutely say Das wäre sehr mutig (That would be very brave). In German, people often use Das to refer back to a whole idea or action they just mentioned.