名词exercise.exercises.

当做体育锻炼时候不加s
练习加s（习题等）

它解释为运动时是不可数名词
If you don't get more exercise, you'll get fat.
解释为习题时才可数
school work exercises

[C]为可数~`即可以加S
[U]为不可数~不加S

n.
1. 运动,锻炼[C][U]
Swimming is one of the healthiest forms of exercise.
游泳是最有益于健康的运动方式之一。
2. 练习;习题[C]
Do the vocabulary exercise at the end of the chapter.
做这一章末尾的词汇练习。
3. 【军】演习;操练[C]
4. (特定的)活动;行动[S][(+in)]
5. (权力,权利等的)行使,运用[S][(+of)]
The government must be careful in its exercise of power.
政府在行使其权力时必须小心谨慎。
6. 【美】仪式,典礼[P]
Graduation exercises are usually held in June.
毕业典礼通常在六月举行。
7. 礼拜;宗教仪式

vt.
1. 锻炼;操练;练习
2. 运用,行使
The King cannot exercise much political power.
这位国王能行使的政治权力并不多。
3. 使忙碌;使担忧;使惊恐
vi.
1. 练习;运动
I think you should exercise more.
我认为你应该多运动运动。

当它作为练习题讲时，可加s
当它作为锻炼、做操讲时，为不可数名词

运动