When does a relative pronoun become the subject of a relative clause and when does it not?

Question

Previously, I thought that a relative pronoun becomes the subject of a relative clause when the relative clause modifies the subject of the main clause. In other words, it serves a double purpose. For example:

Rice, which still forms the staple diet of much of the world's population, grows best in hot, wet lands.

Here, "which" is both the connector and also subject of the relative clause.

However, in other cases, namely when the relative clause does not modify the subject of the main clause, the relative pronoun only becomes the connector of the main clause with the relative clause. For example:

I met the woman whom you mentioned.

Here, "whom" is the connector and "you" is the subject of the relative clause.

But then I encounter the following sentence:

Hail forms within large dense cumulonimbus clouds that develop on hot, humid summer days.

Here, the relative clause does not modify the subject of the main clause. However, "that" becomes the subject as well as the connector of the relative clause.

So, when does a relative pronoun become the subject of a relative clause and when does it not?

Edit:

Please note that this question is different from the one below:

• A relative pronoun acting as both object and subject?

That question asks whether the gap in the following is the object of think or the subject of might

• anything you think might be good for me

The OP there makes no reference to the syntactic function of the anything noun phrase within the larger clause it occurs in.

Answer 1

Its function in the relative clause is completely independent of its antecedent in the main clause. Any clause, whether relative or not, has at least a subject and predicate. Thus, the pronoun can hold any position a noun phrase could regularly take in a clause. It is never just a “connector.” It takes the place of a NP and thus must function in the same way.

Isolate the relative clause and see what function the pronoun performs within it. So if it is the agent performing the verb within the clause, it is the subject; if the action is done to or received by it, it is the direct object (in active constructions; in passive the subject would be on the receiving end); if it follows a preposition, it is the object of the prep; etc. Same rules you would apply to any other clause.

In

Rice, which still forms the staple diet of much of the world's population, grows best in hot, wet lands.

which is the subject of forms.

In

I met the woman whom you mentioned.

whom is the object of mentioned.

In

Hail forms within large dense cumulonimbus clouds that develop on hot, humid summer days.

that, if considered a pronoun, is the subject of develop. (Now, grammarians will disagree and it can be argued that that is a subordinator and not a pronoun, but I won’t get into it here. Read this thread for further detail and justification on the grammar there.)

Still, you can see that the pronoun’s role within the relative clause has nothing to do with whether its antecedent is a subject, object, what have you.