Here are the key moments from the first 2016 presidential debate between Hillary Clinton and Donald Trump on Sept. 26. NBC Nightly News anchor Lester Holt moderated the debate at Hofstra University in New York. (Sarah Parnass/The Washington Post)

Here are the key moments from the first 2016 presidential debate between Hillary Clinton and Donald Trump on Sept. 26. NBC Nightly News anchor Lester Holt moderated the debate at Hofstra University in New York. (Sarah Parnass/The Washington Post)

Donald Trump was trying very hard to be on his best behavior.

In the opening minutes of the first presidential debate Monday night, the Republican nominee began an answer by saying, “In all fairness to Secretary Clinton” — then turned to his opponent with exaggerated cordiality. “Yes? Is that okay? Good. I want you to be very happy. It’s very important to me.”

But even Trump’s best behavior was not quite good enough. Within minutes, he was hectoring and interrupting Clinton when she spoke, glowering, pursing his lips, shaking his head and interjecting one-word retorts.

“Wrong!” he told Clinton.

“Wrong!” he told the moderator, NBC’s Lester Holt.

Donald Trump and Hillary Clinton squared off on the debate stage for the first time at New York's Hofstra University Sept. 26, with NBC Nightly News anchor Lester Holt moderating. (The Washington Post)

“Wrong!” “Wrong!” “Wrong!” he said, over and over again.

But it was Trump who was wrong — on the facts, but also in his approach to the debate.

[Trump bombs on the ultimate reality TV show]

Trump had done well in the primary debates with his insults, interruptions and one-liners. Monday’s 90-minute, one-on-one debate, however, was a format that did not work well for him.

Clinton had crammed for the encounter, practicing heavily. Trump ostentatiously avoided preparation — playing the proverbial high school slacker drinking beer behind the bleachers while the teacher’s pet was in the library.

But Monday night was the revenge of the nerd.

Trump was louder and nastier. But Clinton was cool and measured, continuing to make her case while Trump tried to talk over her. Again and again, she put him on the defensive (“I’m extremely underleveraged,” he assured everybody), and his one-line retorts did not serve him well.

1 of 18 Full Screen Autoplay Close Skip Ad × Clinton and Trump clash during first presidential debate in New York View Photos Republican presidential nominee Donald Trump and Democratic presidential nominee Hillary Clinton face off in the first 2016 presidential debate at Hofstra University in Hempstead, N.Y. Caption Republican presidential nominee Donald Trump and Democratic presidential nominee Hillary Clinton face off in the first 2016 presidential debate at Hofstra University in Hempstead, N.Y. Sept. 26, 2016 Republican presidential nominee Donald Trump and Democratic nominee Hillary Clinton arrive onstage for the first presidential debate at Hofstra University in Hempstead, N.Y. Melina Mara/The Washington Post Buy Photo Wait 1 second to continue.

Clinton speculated that he wasn’t releasing his tax returns because he hadn’t paid income tax for several years.

“It would be squandered,” Trump said, not denying the allegation.

Clinton noted that Trump hadn’t paid federal income taxes for several years previously.

“That makes me smart,” he said.

Clinton accused Trump of rooting for the housing market to collapse in 2006.

“That’s called business,” Trump said.

At one point, Trump took a dig at Clinton because she “decided to stay home,” presumably to practice for the debate. Clinton was ready: “I think Donald just criticized me for preparing for this debate. And, yes, I did. And you know what else I prepared for? I prepared to be president.”

Trump was prepared to do what he has done for 18 months: spout falsehoods. But while that works on Twitter and at campaign rallies, it didn’t go well in the 90-minute debate. Clinton, with an occasional assist from Holt, was able to take some 100 million viewers on a tour of what she called Trump’s “own reality.”

[Last night’s debate, or the mansplaining Olympics]

“Donald thinks that climate change is a hoax perpetrated by the Chinese,” Clinton pointed out.

“I did not!” Trump protested.

He did say so, on Nov. 6, 2012, at 2:15 p.m., on Twitter.

Clinton noted that Trump “supported the invasion of Iraq.”

“Wrong!” Trump declared.

He did, on Sept. 11, 2002, on Howard Stern’s radio show.

Holt mentioned that New York’s stop-and-frisk policy was ruled unconstitutional because it induced racial profiling.

“No, you’re wrong,” Trump told Holt.

Holt was right.

Clinton noted that murders have continued to decline in New York.

“No, you’re wrong.”

Clinton was correct.

Trump repeated the oft-debunked canard that Clinton aides were the first to question Barack Obama’s American birth in 2008. And he dismissed the widely held view among cybersecurity experts that Russia was behind the recent hack of the Democratic National Committee. “It could be somebody sitting on their bed that weighs 400 pounds, okay?”

This was the shot at Trump that Low-Energy Jeb, Little Marco and Lyin’ Ted never had: an extended, one-on-one encounter to test the bilious billionaire. Trump’s playground taunts sounded juvenile.

He declared that Clinton “tells you how to fight ISIS on her website! I don’t think General Douglas MacArthur would like that too much.”

“Well,” Clinton replied, “at least I have a plan to fight ISIS.”

“No wonder you’ve been fighting ISIS your entire adult life,” Trump volleyed.

It was a curious allegation, given that the Islamic State, also called ISIS, is only a few years old and Clinton has been an adult for half a century.

Trump, ignoring Holt’s admonitions, kept up his interruptions: “You didn’t read it! . . . Who gave it that name? . . . Lester, how much?”

When Clinton at one point joked that “I’m going to be blamed for everything that’s ever happened,” Trump interrupted again: “Why not?”

“You know,” Clinton said, “just join the debate by saying more crazy things.”

That he did. He justified his misogynistic words about Rosie O’Donnell by saying that “nobody feels sorry for her.”

Ultimately, insults were not enough.

When Clinton said Trump’s economic plan would add $5 trillion to the debt, Trump interrupted.

“You have no plan!’

“Oh, but I do,” said Clinton, unruffled. “In fact, I have written a book about it.”

“That’s about all you’ve done,” Trump shot back.

And that was about all Trump could do.

Twitter: @Milbank

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