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。