> I don't know **why** **but** for some reason when I got on the train that
> day it was unusually full, something I don't recall ever happening in
> the past.

As for the above sentence, I have some questions:

1. Is why a conjunction or adverb? My own understanding is that *why* is an adverb here which modifies the verb *know*, and *but* is the conjunction which connects two clauses. Is that true?

2. As I read on [Grammarly.com][1], when *but* is joining two independent clauses, we need to put a comma before it. Otherwise, leave the comma out. According to this rule, the two clauses are **'I don't know why'** and **'for some reason when I got on the train that day it was unusually full, something I don't recall ever happening in the past.'**  But since there is no comma and the second clause doesn't express a complete thought, the second clause is a dependent clause or subordinate clause. Is that right?

3. someone told me that the whole sentence is an object clause after *why*, but if the above clues are correct, then the sentence is not a object clause.

  [1]: https://www.grammarly.com/blog/comma-before-but/