The main events are a little difficult to sort out for this story because author Jack Finney weaves the main action events with the main psychological events and with flashbacks used to explain Tom's situation and choices. So in a way, to find the main events, you have to choose between main action events, main psychological events and main explanatory events. Since space is limited here, we'll tackle the main action events.
Of course the main events start with the exposition. Tom has cancelled plans to go to the movies with his wife Clare in order to write an Interoffice Memo about an important independent project of his own design. Tom forces open a stubborn window for air since the room is warm (probably because the building hall is warm). Tom guiltily sends Clare to the movie alone.
Tom forces the front door shut against the rush of warm hall air into the cool air of the apartment. He turns to see papers fluttering to the ground and the all-important yellow sheet of paper, with the shorthand notes of his project idea on it, sail out the window.
When Tom see the paper is stuck at the corner of the ledge, he eventually rejects all ideas but the idea to climb out along the ledge to retrieve it. He climbs out with stomach, chest and face pressed to the brick wall. These are the main events that lead Tom to the adventure and terror of a lifetime.
After inching along the ledge, Tom dangerously bends down to successfully grab the paper. Unfortunately, while bent, he sees down the heights to Lexington Avenue far below and Loew's theater in the distance. He is overcome with terror, weakness, faintness and becomes paralyzed, without courage and unable to move. In a massive exertion, Tom begins to move again, this time back to the window. Between here and the window, Tom goes through many psychological events and there are explanations, but we'll confine this to actions.
He arrives at the window. He is thrown off by the emptiness his fingers encounter instead of brick wall. He nearly falls off, instead he drops to his knees pulling the open window shut with his falling weight.
he was kneeling on the ledge, his forehead pressed to the glass of the closed window.
Dropping his palms to the sill, he stared into his living room--at the red-brown davenport across the room, ... and at his papers, typewriter, and desk, not two feet from his nose.
From this position he must break the glass to get in. Many actions--envelopes lit as signals, coins dropped below to hoped for passersby, yells--had failed to gather notice; he must break the glass. Tom ultimately succeeds in driving a mighty fist-punch through the glass and falls part way inward. After picking away the glass shards, he crawls into his apartment.
In shock, he lays the yellow paper on the desk where it had been, weights it down ineffectually with a pencil, goes to the closet to get his topcoat and hat, then exits to find Clare at Loew's. As he pulls the door shut behind him, he sees the pencil fly and the yellow sheet of paper flutter out the open window. Tom laughs in ironic derision and disbelief. He goes to find Clare.