An 'A-ha' moment in 'How I Met My Husband' would be the moment Edie realizes that she was never going to get a letter from Chris.
When Edie becomes infatuated with the former military pilot, she starts to dream of possibilities with him. Edie (who works as a household helper to Dr. and Mrs. Peebles) meets Chris when he asks for some water from the Peebles' pump. One evening, after checking on the children, she steals over to see Chris at the local fairgrounds, where he gives plane rides. Infatuated with the suave airman, the young teenager doesn't realize that the older man cannot pursue a relationship with her.
When Alice Kelling, who supposes herself Chris' fiance, comes calling for him, Edie is unimpressed. She thinks the woman isn't very attractive and that her engagement ring is quite a bit smaller than the one Muriel Lowe's cousin wears.
Meanwhile, Chris engages in some affectionate kissing with Edie in his tent. Edie, who has never been initiated into any sort of sexual intimacy, is thrilled. She believes him when he tells her that he is going to write her a letter to let her know his whereabouts and that, perhaps, she can come to see him. Chris then flies away without informing Alice or anyone else.
Edie revels in what she thinks is her secret pact with Chris. However, when the jealous Alice insinuates that Edie and Chris have been sexually intimate, Edie bursts into tears. It takes some coaxing from Mrs. Peebles to make Edie realize that kissing does not constitute sexual intimacy or intercourse (an 'a-ha' moment for her).
The next 'a-ha' moment is when Edie finally realizes with each trip to the mailbox that a letter from Chris was never going to materialize. The story ends happily: Edie becomes engaged to and finally marries the mailman.