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I. it／one ／that三者均可用作代词, 指代前面提到的名词。一般说来, it指代同名同物; one与that则指代同名异物。
I have lost my umbrella; I'm looking for it. （该句中it就是指前面的my umbrella）
I have lost my umbrella; I think I must buy one. （one在该句中表泛指, 因为my umbrella已经丢了）
The umbrella you bought is cheaper than that I bought. （替代词that在该句中特指“the umbrella I bought”, 以区别“the umbrella you bought”）
II. one与that虽可用来指代同名异物, 但one为泛指, 相当于a／an＋名词; that为特指, 相当于the ＋名词。所以one所指代的名词的修饰语一般为 a／an ／some ／any; that所指代的名词的修饰语往往是the ／this ／that。
A chair made of steel is stronger than one made of wood. （该句中one可以换成a chair）
The water in the cup is hotter than that in the pot.
one既可代替事物, 也可代替人, that只能代替事物而不能代替人。有时可以用the one或the ones代替that或those。
The one ／That on the table is mine. （该句中The one代替事物, 并且也可以用That）
He is the teacher, the one who is loved by the students. （该句中the one代替人, 不能用that）
He advised the farmers to choose the best seed-heads, the ones ／those that had the best color. （该句中the ones代替事物, 并且也可以用those）
