Where is the subject in this sentence?

AnonymousThe mountains

That is the subject of the sentence.

Sir, and how do you call "I" then? And what is "I have climbed"?
Thanks for an answer!

'I' is the subject of the subordinate, relative clause modifying 'mountains'; it has an elided relative pronoun: The mountains [that] I have climbed help me enjoy the fall.

The main clause is this: The mountains help me enjoy the fall.
A subordinate relative clause can be inserted. It further restricts the subject (mountains): The mountains that I have climbed help me enjoy the fall.
The relative pronoun is sometimes dropped: The mountains I have climbed help me enjo

I’d say the main clause is the entire sentence. The relative clause is restrictive and therefore an indispensable part of the noun phrase it defines, so the subject is the mountains [that] I have climbed.