He said he couldn't follow me.句中couldn't能换成can't吗?

He said he couldn't follow me.句中couldn't能换成can't吗?
He said he couldn't follow me.他说他跟不上我
表示过去的能力时只能用can的过去式could吗?我查了资料又说了许多表示过去能力不用could的情况,感觉好乱.
甚至有些语法直接说二者没有时态区别!