prepositions - "... that ... with ..." and "... which ... with ..."

1. The fact that I communicated to Mona is irrelevant.


2. The fact that I communicated with Mona is irrelevant.

The only difference might seem to be the different prepositions, with and to.

Now, try replacing the that in each sentence by which. Do you agree that you can do it with 1) but not 2)? If so, what exactly is wrong with "The fact which I communicated with Mona is irrelevant"?

Answer

The short answer: that can be a conjunction or a relative pronoun; only when it is a relative pronoun can it be replaced with which.

Your first example could mean either of the following:

a.) This fact, which I communicated to Mona, is irrelevant — relative pronoun.

b.) The fact that I communicated something to Mona is irrelevant — conjunction.

If we read it as a), the word that in your example has a function within the predicate of the subordinate clause that I communicated to Mona: it is the object of communicated. What did I communicate? I communicated the fact ( = the antecedent of that).

If we read it as b), that just serves to introduce the content of the subordinate clause, but it does not itself have a function within the predicate of the clause: that's why it is called a conjunction.

In your second example, however, the preposition with makes it unlikely that an object is present: you would normally not say, ?I communicated this with Mona. For that reason, we automatically read that as a conjunction here, not a relative pronoun; and only relative pronouns can be replaced with which.