the old man is so old .he can't walk fast合为一句

The old man is too old to walk fast. (too...to ...结构,表示“太...以至于不能.”)
The old man is so old that he can't walk fast. （so...that...结构,表示“如此...以至于...”）
注意区别：1句句型已带有否定含义,所以后不加not,2句不带否定含义,所以还要加上not.
希望好好学习英语啊,加油!