13. It's easy to keep cacti.
仙人掌很好养活。
还能这样说：
Cacti can be easily kept.
It's not difficult to keep cacti alive.
应用解析：
in bad keep 保存得不好；
in good keep 保存得好；
keep from 避开；禁止；隐瞒；
keep on with 继续
14. Can you give me some advice about how to plant camellia?
关于养茶花你能提些建议吗？
还能这样说：
Could you tell me how to grow camellia well?
Can you teach the methods of growing camellia?
谚语：
Many receive advice only the wise profit by it.
得到忠告的人很多，唯智者能从中受益。
15. We have a grassy lawn in our garden.
我们的花园里有绿草如茵的草坪。
还能这样说：
You can see a green turf in our garden.
Our garden is covered with grassy lawn.
谚语：
The neighbor's lawn is green.
邻家的草坪绿（老婆别人的好）。
16. Keep off the grass, please!
请勿践踏草坪！
还能这样说：
You're not supposed to walk on the grass.
You are not allowed to walk on the lawn.
应用解析：
cut one's own grass 自食其力；
cut the grass from under sb.'s feet 妨碍某人；拆某人的台；
hear the grass grow 特别敏锐
17. The flowers have withered away.
花谢了。
还能这样说：
The flowers have fallen and scattered about.
The flowers have sicken.
应用解析：
the flower of the youth of the country 国家的优秀青年；
no flowers (by request) 花圈敬辞；
as welcome as flowers in May 极受欢迎的