情态助动词can的使用规则

Can 没有任何变形，主语是第三人称单数（he/she/it）也不会变化。

Can I have the menu? 能给我菜单吗？
Can you pass me the newspaper? 你能把报纸递给我吗？
Can she sit next to me? 她能坐在我旁边吗？
Can we pay you now? 我们可以现在付钱给你吗？
Can they use the VIP lounge? 他们可以使用贵宾休息室吗？（VIP lounge 贵宾休息室，lounge 休息厅，等候室）

Can后面跟动词原形。
Can I have the menu?
Can you pass me the newspaper?
Can she sit next to me?
I can hear you.

疑问句直接把can提前，放主语前面，不需要用do或does。

简短回答
Yes, you can / No, you can't
Yes, s/he can / No, s/he can't
Yes, they can / No, they can't

否定形式是can't，不需要用don't或doesn't。

I can't believe you said that! 
我不敢相信你那么说！
said 是 say 的过去式，say 说，讲

Can't的完整形式是cannot，在正式语境或想强调某事时用cannot。
I cannot understand why she behaves like that.
我不明白她为什么那样做。
behave 表现，行事
like that 那样地（的）