[Grammar] one v.s. it

[sitifan]

Mrs. Yeats knew a spade when she saw it.
https://books.google.com.tw/books?id=1lZdDwAAQBAJ&pg=PA124&dq=%22knew+a+spade+when%22&hl=en&sa=X&ved=0ahUKEwi5gZvlxvjjAhWRLqYKHbapAyUQ6AEIJzAA#v=onepage&q=%22knew%20a%20spade%20when%22&f=false
He came of a line of people who knew a spade when they saw one.
(Joint College Entrance Examination in Taiwan, 1983)

In the above sentences, are "one" and "it" interchangeable?

 

[jutfrank]

Without affecting the basic meaning, yes. Both pronouns make reference to the same thing.

But remember that you cannot interchange any two words without changing something.

 

[abaka]

The "it" should refer to a particular spade; "one" to an abstract or non-specific one.

In practice, though, the two pronouns are so close that you'll often see or hear one or the other, both ways.