when does clark tell lana

Clark directly tells Lana his full secret twice in Smallville :

• First, in Season 5, Episode 12 – “Reckoning” , he takes her to the Fortress of Solitude and reveals his powers and that he’s not human, then proposes. This timeline is later erased, so she loses that knowledge.

• For good in the main timeline, she finds out and he confirms everything by the end of Season 6 : she sees his powers in “Promise” (Season 6, Episode 16) , then in the Season 6 finale “Phantom” (Episode 22) he explicitly tells her he’s an alien from Krypton and explains his origin.

So if you’re watching through:

• “Reckoning” is the big first reveal (but gets undone).

• “Promise” + “Phantom” are where Lana truly learns and keeps the truth for good.