NEW YORK — The judge in Donald Trump’s hush money trial fined him $1,000 on Monday and, in his sternest warning yet, told the former president that future gag order violations could send him to jail.

The reprimand opened a revelatory day of testimony, as jurors for the first time heard the details of the financial transactions at the center of the case and saw payment checks bearing Trump’s signature.

The testimony from former Trump Organization controller Jeffrey McConney provided a mechanical but vital recitation of how the company reimbursed payments that were allegedly meant to suppress embarrassing stories from surfacing during Trump’s 2016 presidential campaign and then logged them as legal expenses in a manner that Manhattan prosecutors say broke the law.

McConney’s appearance on the witness stand came as the first criminal trial of a former U.S. president entered its third week of testimony. His account lacked the human drama offered Friday by longtime Trump aide Hope Hicks, but it nonetheless yielded an important building block for prosecutors trying to pull back the curtain on what they say was a corporate records cover-up of transactions designed to protect Trump’s presidential bid during a pivotal stretch of the race.

At the center of the testimony was a $130,000 payment Trump’s then-lawyer and fixer Michael Cohen made to porn actor Stormy Daniels in October 2016 to stifle her claims of an extramarital sexual encounter with Trump a decade earlier.

The 34 felony counts of falsifying business records accuse Trump of labeling the money paid to Cohen in his company’s records as legal fees. Prosecutors contend that by paying him income and giving him extra to account for taxes in monthly installments for a year, the Trump executives were able to conceal the reimbursement.

McConney and another witness testified that all but two of the monthly checks were drawn from Trump’s personal account. Yet even as jurors saw the checks and other documentary evidence, prosecutors did not elicit testimony Monday showing that Trump himself dictated that the payments would be logged as legal expenses — a designation that prosecutors contend was intentionally deceptive.