What does Montag suddenly remember?
Short answer: Montag suddenly remembers that he met Mildred in Chicago.
When Montag reaches the community of readers who have memorized books in hopes of preserving them for a time that will come when people will be ready again to learn from ages past, he tells some of the people that he has left his wife Mildred behind, but he does not miss her, probably because she has betrayed him. Later, he hears the planes and the first bomb strikes. Montag shouts "Mildred!" In his mind, he sees her falling, hears her screaming in the hotel where she has gone when their house was burned. The idea of the hotel triggers his memory:
I remember. Montag clung to the earth. I remember. Chicago. Chicago a long time ago. Millie and I. That's where we met! I remember now. Chicago. A long time ago.
Then, the earth shakes from the bombs. Montag crushes himself as small as he can, and as he feels the grit and dust, he remembers something else, too. It is part of Ecclesiastes. It is about there being a time for all things.
At the beginning of the novel, Montag meets his charming, unique teenage neighbor, Clarisse. Montag and Clarisse have an interesting conversation, and she asks Montag if he is happy before they head their separate ways. This question bothers Montag as he begins to contemplate whether or not he is living a happy life. When Montag enters his home, he looks up to the ventilator grille in the hall and suddenly remembers that something is hidden behind the grille. Montag then quickly looks away and continues to think about his meeting with Clarisse. Although Bradbury does not immediately reveal what is hidden behind the grille, the reader eventually learns that Montag has been hiding a stash of illegal books in his ventilator. After Captain Beatty visits Montag's home, Montag takes the books out of the ventilator and attempts to read them. Unfortunately, Montag cannot comprehend the texts and seeks the help of Faber.