Consider the two sentences:
Whenever I discuss X, people don't know what I'm talking about.
Whenever I discuss X, people don't know what I talk about.
I think the first one should be obviously idiomatic to any anglophone, while the second one sounds unnatural, ungrammatical, and wrong. But why?
"Whenever" implies regularity, a repeated action, so we use the Present Simple tense. Hence, it should be "discuss", "know", and "talk". Instead, "talking" is idiomatic.
Can anyone provide a good grammatical rationale for this phenomenon? I would appreciate any authoritative references since I'm drawing a blank.