13. When I was a child, I used to jump rope.
我小的时候常常跳绳。
还能这样说：
When I was young, I used to skip rope.
When I was in the childhood, I always skipped rope.
应用解析：
jump to it 赶快；
jump the gun 过早地行动；
get the jump of sb. 得到超越某人的优势；
give sb. a jump 吓某人一跳；
keep one jump ahead of sb. 保持领先于对手的地位
14. Will you jump rope with me?
你愿意和我一起跳绳吗？
还能这样说：
Are you willing to skip rope with me?
Shall we jump rope together?
15. It is very interested to watch them jump rope.
看他们跳绳很有意思。
还能这样说：
It is full of fun to watch them skip rope.
Watching them skip rope is amusing.
应用解析：
watch one's step 谨慎从事；
watch the clock 工作时看表，心不在焉地工作；
keep a close watch on 密切关注；
on the watch for 警戒，注意