ascend 正式用词，指不用手攀，一直上升直到相当高的地方。
climb 普通用词，含义广泛，侧重运用手足费力地攀登或上升，也可指抽象事物。
mount 书面用词，词义与ascend相近，强调连续不断地向上移动、攀登或上涨。指骑马时用mount。
参考例句
1. The old lady climbed up the stairs with difficulty. 这位老太婆艰难地爬上楼梯。
2. I want to climb the mountain. 下次休假时。我想去爬山。
3. No more trunk for you to climb on. 树又说：“我再没有树干，让你爬上来了。”
英英解释
n.
1. an upward slope or grade (as in a road)
2. an event that involves rising to a higher point (as in altitude or temperature or intensity etc.)
3. the act of climbing something v.
1. go upward with gradual or continuous progress
2. move with difficulty, by grasping
3. go up or advance
4. slope upward
5. improve one's social status
6. increase in value or to a higher point
