Walking is good exercise.为什么不用加冠词a啊?

walking is good exercise.为什么不加a? Walking is good exercise,为什么walk要加ing? It's good exercise.为什么good前不用加a? Walking is good exercise.同意句 she is my good friend 为什么不要加冠词 “a” fridend 不是可数名词么?不懂 Walking up and down the stairs is a good kind of exercise to Walking is a fine exercise同义句 this is good exercise 为什么不是this is a good exercise ? i think walking is good for my health walking为什么加ing Walking is good中Walk为什么要加ing呢还有就是It is a fine day for walkin 为什么这个句子不用加冠词? 为什么有些名词前面不用加冠词