Learn Hardcore Russian: Я не помню, откуда я узнал этот секрет. - I don’t remember where I learned this secret.

In Russian, помнить means “to remember” (have something in your memory), whereas знать means “to know” (possess knowledge or information).
• Я не помню = “I don’t recall,” “I can’t remember.”
• Я не знаю = “I don’t know.”
Here the speaker isn’t saying “I don’t know the secret,” but “I don’t remember from where I learned it.”

What exactly does откуда mean and why is it used here?

откуда is an interrogative adverb meaning “from where.” In this sentence it introduces an indirect question (a subordinate clause).
• Direct question: Откуда ты узнал этот секрет? (“Where did you learn this secret from?”)
• Indirect question: Я не помню, откуда я узнал этот секрет.
No case ending is needed because откуда isn’t a noun—it’s an adverb.

Why is there a comma before откуда?

Russian punctuation requires a comma to separate the main clause from a subordinate clause introduced by an interrogative word like откуда.
Structure:
Main clause (“Я не помню”) , Subordinate clause (“откуда я узнал этот секрет”).

Why is the verb узнал in the perfective aspect?

The perfective verb узнал (from узнать) indicates a single, completed action in the past: “I learned/discovered the secret.”
If you used the imperfective узнавал, it would imply a repeated or ongoing process (“I was learning the secret again and again”), which doesn’t fit here.

Why is этот секрет in the accusative case even though it looks like the nominative?

секрет is masculine and inanimate. In Russian, the accusative form of a masculine inanimate noun is identical to its nominative. Grammatically, этот секрет is the direct object of узнал, so it takes the accusative case.

Could you say Я не помню, откуда я узнал об этом секрете? What’s the difference?

Yes, you can say узнал об этом секрете (from узнать о чём) — “found out about the secret.”
• узнал этот секрет = “got to know the content of the secret,” “learned the secret itself.”
• узнал об этом секрете = “became aware that such a secret exists,” without necessarily knowing its details.

Why is the main verb in the present tense (не помню) while the subordinate verb is in the past (узнал)?

Russian doesn’t require tense agreement between clauses the way English sometimes does. Here:
• Present tense in the main clause expresses the speaker’s current state (“I don’t remember right now”).
• Past tense in the subordinate clause refers to the completed action that took place earlier (“when I learned it in the past”).
This combination is perfectly natural.

Could we change the word order and say Я не помню, я узнал этот секрет откуда?

That word order would be extremely awkward or ungrammatical. In Russian indirect questions introduced by откуда, the interrogative word normally comes first:
Correct: Я не помню, откуда я узнал этот секрет.
Moving откуда to the end would turn it into an afterthought and break the indirect-question structure.