1.I always go roller skating and I always happy.

错误处用括号标出：
1.I always go roller skating and I (加入am) always happy.
and说明后面是一个句子,句子不能缺少谓语动词be,根据时态和主语I选用am.
2.You can drink lots of water (without) getting fat.
根据句意用without:你可以喝很多水,而不会变胖.
光喝水当然不会变胖.
3.People in the 1990's didn't wear this kind of trousers.
该句应该没有错误.其中：in the 1990's = in the 1990s “在二十世纪九十年代”
4.The skirt is too expensive.I don't have enough money to buy (it).
it指代前句中的the skirt,为单数,但是them指代的是名词复数形式.
5.(Eating) good food and doing exercise help you stay (healthy).
动词原形不能做主语,改用动名词以和后面的doing保持一致；
stay healthy是系表结构,所以用形容词形式作表语.

大哥，你好好学习不行吗~

1. I always go roller skating and I always be happy
2. You can drink lots of water without getting fat
3. People in the 1990s didn't wear this kind of trousers
4. The skirt is too expensive.I don't have enough money to buy it
5. Eating good food and doing exercise help you stay health
呵呵

1. I always go roller skating and I always be happy.
2. You can drink lots of water without getting fat.
3. People in the 1990s didn't wear this kind of trousers.
4. The skirt is too expensive.I don't have enough money to buy it.
5. Eating good food and doing exercise help you stay health.