13. Li Ning is my favorite gymnast.
李宁是我最喜欢的体操运动员。
还能这样说：
Among all the gymnasts, I like Li Ning best.
My favorite gymnast is Li Ning.
14. The students must practise gymnastics on the playground in the morning.
学生早上必须在操场上做操。
还能这样说：
The students are obliged to do gymnastics on the sports ground in the morning.
The students are ordered to practise gymnastics on the sports field in the morning.
应用解析：
the morning after the night before 宿醉；
morning glory 牵牛花；
the morning of life 生命之初期，青少年时期；
toward morning 接近早晨时
15. Children are forbidden to do some dangerous gymnastic performances.
不允许孩子们做一些危险的体操动作。
还能这样说：
People don't allow children to do some dangerous gymnastics.
Children should be kept far away from some dangerous gymnastic performances.
应用解析：
a child prodigy 神童，天才儿童；
child labor 童工；
child memories 孩提时的回忆；
with child 怀孕的；
a child of fortune 命运的宠儿；
a child of nature 自然之子，天真无邪的人
16. Is this your very first time to do gymnastics?
这是你第一次做体操吗？
还能这样说：
Do you practise gymnastics for the first time?
Do you just start to do gymnastics?
应用解析：
first and foremost 首先，第一；
at first glance 乍看之下；
not have the first idea about sth. 对某事一无所知；
first hand 一手的，直接从源处获得的；
first lady 总统夫人，州长夫人
17. I haven't taken exercise for a long time.
我很久没运动了。
还能这样说：
It has been a long time for my last exercise.
Doing exercises has faded out of my life for a good while.
应用解析：
to cut a long story short 长话短说，简而言之，总之；
take a long view 从长远考虑事情、效果等；
take a long look at 谨慎仔细考虑；
not by a long shot 完全不，决不；
in the long run 从长远看，最后，终究