This Short Story was first published in Fantastic (Summer 1952 issue), by Isaac Asimov (and then republished in the June 1969 issue). Norman and Liwy (Olivia) meet an unusual man on the train.
To celebrate their five years of marriage, Norman and Liwy take a train to New York for a week long holiday. While on the trip, Liwy expresses doubt about their relationship, concerned that a small change in their past would result in living completely different lives. Norman disagrees, believing that no change could prevent them from deciding to marry. They're lucky enough to meet a man who can show them exactly what would've happened if things were different.
You may be looking for a trope called What If?, or another work by the same name.
"What If—" was republished ten times;Science Fiction Adventures In Dimension (1953), Adventures In Dimension (1955), Great Science Fiction By Scientists (1962), Nightfall and Other Stories (1969), Sirius (issue #22, April 1978), Isaac Asimovs Magical Worlds Of Fantasy 07: Magical Wishes (1986), Fantastic Stories: Tales of the Weird & Wondrous (1987), Isaac Asimov: The Complete Stories, Volume 1 (1990), Time Machines (1997), and The Time Travellers Almanac (2013).
"What If—" contains examples of:
- Alternate Universe: Mr If's scrying glass allows people to view "what might have been". Liwy and Norman ask "What if we hadn't met that day?" and see events play out slightly differently because of it.
- Crash-Into Hello: Liwy and Georgette have boarded a streetcar where Norman is sitting. When they get near him, the streetcar swerves, and in one timeline, Liwy falls into his lap, causing them to marry in six months. In the other, she catches herself and he marries Georgette instead.
- Crystal Ball: Mr If has a foggy plane of glass that shows a clear image when you ask it a question about "what might have been?" While our protagonists are watching, they suddenly find themselves experiencing the scene directly, as if it was a Flash Back.
- Description in the Mirror: When the strange little Traveling Salesman shows up in their train car, Liwy tries to ignore him and pulls out her mirror to inspect her appearance. This is when the reader learns she has brown hair and blue eyes.
- Driving Question: The text tells us that Liwy's favourite question is "What if Liwy and Norman hadn't met on the streetcar?" Mr What If helps them to explore that exact Alternate Universe, showing that no change could prevent them from deciding to marry.
- Flash Back: Mr What If has a pane of cloudy glass that he uses to show "what if" timelines. The characters experience them as if they were reliving a memory together.
- For Want Of A Nail: When Mr What If shows Norman and Liwy what life would've been like if they hadn't met on a streetcar, they find that they got engaged to completely different people. Norman had even married Georgette, while Liwy was dating another of their friends.
- Insatiable Newlyweds: In the "what if" where Norman married Georgette, the "present day" has Norman and Liwy celebrating their recent wedding. He is much more interested in the physical expression of his love compared to the timeline where it has been five years since they married.
- In Spite of a Nail: When Mr What If shows Norman and Liwy what life would've been like if they hadn't met on a streetcar, they find that some things don't change at all. Norman still spills a drink on Liwy's dress, and their "today" would still be spent on a train to New York, celebrating their marriage.
- The Little Shop That Wasn't There Yesterday: A rare Traveling Salesman example; when Mr What If has finished showing the protagonists an Alternate Universe where they didn't meet on the streetcar, he completely disappears, as if he hadn't been there to begin with.Liwy said with sudden sharpness, "Where's Mr. If? [...] he wouldn't leave his hat." And she bent to pick it up.
Norman said, "What hat?"
And Liwy stopped her fingers hovering over nothingness. She said, "It was here-I almost touched it." - New Year Has Come: The first New Year's celebration after their wedding, Norman accidentally spilled his drink on Liwy and they had to leave early. When they see what might've happened in the timeline where he marries Georgette, he still spills it on Liwy.
- The Speechless: Mr What If never speaks during the story, but still effectively explains his name and what he's capable of doing.
- Traveling Salesman: This story takes place when travelling salesmen were a common sight. Like many of his contemporaries, Mr What If carries a large suitcase emblazoned with his name/job. Unlike many of his contemporaries, Mr If literally disappears when he's finished the job.
