Hillary Clinton will be there. Donald J. Trump, too. Pleasantries will probably be exchanged. And after that, the road map for the first presidential debate is anyone’s guess. What are some of the most consequential “what-ifs” ahead of Monday night’s showdown — the possibilities that make the campaigns sweat? Or should:

What if Trump invokes Bill Clinton’s infidelities?

He is off to a head start: Mr. Trump already threatened to invite Gennifer Flowers, who claimed to have had a 12-year affair with Bill Clinton, as his debate guest. (Mr. Trump’s campaign later said that Ms. Flowers would not be there.)

Still, a decision to confront Mrs. Clinton from the stage about her husband’s misdeeds would raise the stakes considerably. Mr. Trump plainly sees a benefit in reminding voters of past Clinton scandals they may have forgotten. But the risks are substantial: The move could make Mrs. Clinton appear more sympathetic and repel female voters already uneasy with Mr. Trump.

Then there is Mr. Trump’s marital history to consider.

What if Clinton drops a pop quiz?

No one likes a know-it-all. But voters, presumably, want their president to know at least a few things. Among some Republicans who hoped to stop Mr. Trump in the primaries — and who remain convinced that he could not survive a middle-school civics class — a great regret persists: No one ever tried to insult his intelligence by stumping him with an easy question.

What is the Common Core, which he claims to despise? Who is the prime minister of Canada? How does a bill become a law?