1. I think it hard to learn gymnastics.
我认为学体操很难。
还能这样说：
I think it difficult to learn gyms.
To me, it's not easy to learn gyms.
谚语：
A faithful friend is hard to find.
知音难觅。
2. I like to do calisthenics.
我喜欢做健美操。
还能这样说：
I am fond of aerobics.
Calisthenics is my favorite.
3. The first thing before I have my breakfast is to do exercises.
我吃早饭前的第一件事是做操。
还能这样说：
I do exercises before my breakfast.
I don't have my breakfast until I finish doing gyms.
谚语：
Cast not the first stone.
责人之前先自省。
4. Let's go and do exercises.
大家去做操吧。
还能这样说：
It's time to do gym.
What about doing exercises?
应用解析：
let sb./sth. be 不干涉，不打扰;
let go of 松开，释放;
let us say 比如;
to let 待租，出租