SO & SO THAT it is adj to ....

Q1) i am fat so i cannot run fast
i am so fat that i cannot run fast
有無分別
有咩分別

A1) 有分別

I am fat so I cannot run fast == 整句主要表示因果關係；意思為「我肥胖所以不能跑得快」。這一句的「I cannot run fast」是整句主要要表達的信息，而「I am fat」則是用作解釋為何不能跑得快。

I am so fat that I cannot run fast == 整句主要是突出「我很肥胖」；意思為「我肥胖得不能跑得快」。這一句的「I cannot run fast」是用作具體說明「I am so fat」，在這句中「I am so fat」才是整句主要要表達的信息。

Q2) it is easy for John to cook rice
John is easy to cook rice
rice is easy to cook
John cook rice easily
有無分別
有咩分別

A2) 第一句和第四句的意思一樣，沒有分別
i.e. 「it is easy for John to cook rice」= 「John cook rice easily」 = 煮飯對John來說十分容易

注意！第四句的Grammar 應是「John cookS rice easily」;

第三句「rice is easy to cook」係一句General statement，意思指「煮飯是一件容易的事」。

第二句的Grammar 是錯的！不應這樣寫。可考慮改成第四句 / 第一句。

Q3) it is careless of Tom to forget...
Tom is careless to forget...
有無分別
有咩分別

A3) 意思上沒有分別，係Sentence Structure 的分別，第一句是Passive voice 「被動式」，第二句是Active voice 「主動式」

I am fat, so I cannot run fast. If there is a comma, it is easy to explain the first sentence.
It means,"The reason for my being unable to run fast is that I am fat".

The adverb "so" in the second sentence is clear. The whole sentence means," I am not just fat; I am very fat, so much so that I cannot run fast."

"It is easy for John to cook rice." and "John cook rice easily" are the same.
"Rice is easy to cook." for everyone. " For John, rice is easy to cook." is all right.
"John is easy to cook rice." is a wrong sentence. It may be changed to "John finds it easy to cook rice."

There is a slight difference in meaning in beginning your sentence in the last two ways:
The first means Tom should not forget, but this time he does, because he is being careless. The second is a plain statement about how Tom is careless to forget something.

2008-02-24 23:11:52 補充：
In the 4th para. the sentence should be "John cooks rice easily".