下面这句话中的“so”能不能替换为“so that”？二者在表示“因此”时有什么区别？可以互换吗？

直接给你举一个 so Vs. so that 的例子你自己体会吧
I am eating now so that I don't get hungry later.

I don't want to get hungry later, so I am eating now.
意思上的区别: so that 是用来解释intension的, 也就是你做某件事的意图. so 则是解释 effect, 也就是因为某种需要而产生的后果.
语法上的区别: so 必须在逗号后面且在两个独立句之间. so that没有这样的硬性要求.
如果这句话你想用 so that 的话, 你还得加个的意图, 比如
Since there is only enough spring water to supply the needs of the house, we have to pump from the river for farm use so that we don't exhaust the water supply for the house.
但是这样明显句子就繁琐很多, 而且后面这个意图不用明说, 通过前面的前景设定大家也自然能想到, 所以干脆只用 so.

而至于你说的 so that 前面一般有个 have to 这是个表象. 你要透过现象看本质... I have to do something so that I can achieve some goal. 就是 so that 在表达意图. 而表达意图的行为一般用 I have to do something... 但也可以是任何其他行为的表达式. 所以不能只靠有没有 have to 来判断用不用 so that

so是表示结果的相当于所以，so that是表示目的的，所以不能互换。

So that

1. you can use"so that"especially with can,could,will and would

We hurried so that we wouldn't be late.(=because we didn't want to be late)

I learn English so that I can talk to native speakers.

2.You can leave out that, so you can say:

We hurried so we wouldn't be late.

I learn English so I can talk to native speakers.

(Note that there are no commas before so that or so in these sentences)

看你问一大堆的 新概念英语的题目，你为什么不去英语答疑网问呢，我碰到语法怎么都发那里