can为情态动词，跟主语是第几人称没关系；
can作为情态动词，后面是加动词原型的。
When can I get my test results?
我什么时候可以拿到化验结果？
We can reduce the problem to two main issues.
我们可以将这个问题概括成两个要点。
The operation could prolong his life by two or three years.
这次手术可使他多活两三年。
