Hussein Hassan posted:- I've never climbed a mountain that _________ ( is / was ) dangerous.
Can both be used?
Hi, Hussein: Yes, both can be used. Ms. Tan is overlooking the possibility that your not having climbed a dangerous mountain to date does not exclude the possibility that a mountain that you have climbed was dangerous at some time preceding your climb.
I actually find "was" to be the better choice and the sentence itself to be a bit awkward. The intended meaning is probably that the speaker has never climbed a mountain that was dangerous to climb, since no mountain is inherently dangerous. You could even add expletive "it":
- I've never climbed a mountain that it was dangerous (for me) to climb.