In this sentence:
The diet could be prescribed for someone with any disease in which there is an abnormal retention of fluid.
Is it correct to replace "in which" with "in that"?
Where we could do that?
Are they interchangeable?
Can we put prepositions exactly next to "that" (as a relative pronoun)?
like this:
He worked for a spy network about that I knew nothing.
That cannot head a relative clause when it is the object of a preposition which it directly follows—that is, when the preposition is 'pied-piped' with the relativizer to the beginning of the clause:
He worked for a spy network about ∗that I knew nothing. This must be written using which . . .
He worked for a spy network about okwhich I knew nothing.
In ordinary speech, that can head a relative clause if the preposition is not pied-piped but 'stranded' in its original position:
He worked for a spy network ok?that I knew nothing about.
Formal written English frowns on this; but formal written English frowns on stranding the preposition, too, although that's now far more common than it used to be.
And you should note that neither "rule" applies when the relativizer (which or that) is not the object of the preposition:
He took off his hat. This is the hat okthat he took off.
In this case hat is not the object of off but the Direct Object of take, and off is ...
In any case, it is not a preposition governing the relativizer, whether that's that or which, so it cannot be pied-piped—
This is the hat ∗off which he took.
It must remain stranded in its original position.
