Originally Published: 02 AUG 22 09:55 ET

By Dakin Andone and Kevin Conlon, CNN

(CNN) -- It's been 1,630 days since Linda Beigel Schulman spoke to her son Scott Beigel, a geography teacher killed in the 2018 high school shooting in Parkland, Florida, as he ushered students to safety in his classroom.

"I will never get over it. I will never get past it," she said Monday, testifying in the gunman's death penalty trial in a Broward County courtroom. "My life will never, ever be the same."

Beigel Schulman was among the first of the victims' loved ones to take the stand Monday in the death penalty trial of shooter Nikolas Cruz, along with relatives of Joaquin Oliver and Alaina Petty, students who, along with Beigel, were among 17 people gunned down on Valentine's Day 2018 in the shooting at Marjory Stoneman Douglas High School.

"It's been four years and four months since he was taken from us, his friends and his family," Patricia Oliver said of her son, who was 17 when he was killed. "We miss him more than words can say and love him dearly," she said, adding, "Our lives have been shattered and changed forever."

Jurors are expected to hear testimony Tuesday from more of the victims' loved ones as they take the stand to help illustrate the toll the murders have taken.

Cruz, now 23, pleaded guilty in October to 17 counts of murder and 17 counts of attempted murder, and this phase of his criminal trial aims to determine his sentence: Prosecutors are seeking the death penalty, while Cruz's defense attorneys are asking the jury for a sentence of life in prison without the possibility of parole.

To recommend a death sentence, jurors must be unanimous. If they do so, the judge could choose to follow the recommendation or sentence Cruz to life instead.

To make their decision, jurors will hear prosecutors and defense attorneys argue aggravating factors and mitigating circumstances -- reasons Cruz should or should not be executed. Victim impact statements add another layer, giving the families and friends of the victims their own day in court.

"We don't have a system where it's the victims' families that get to decide whether you live or die if you kill their family members," Teresa Reid, legal skills professor at the University of Florida's Levin College of Law, has told CNN. "We don't have revenge. And so this is the mechanism that the family has."

Cruz had no visible reaction Monday to any of the victim impact statements, though one of his defense attorneys was seen wiping away a tear, as were at least two members of the jury.

Joaquin's sister, Andrea Ghersi, said her 6-foot-1 baby brother was "energetic, vibrant, loud, confident, strong, empathetic, understanding, smart, passionate, outgoing, playful, loving, competitive, rebellious, funny, loyal and constantly spoke up when he felt something was not just."

Victoria Gonzalez also took the stand Tuesday. The day of the shooting, she became Joaquin's girlfriend, Gonzalez told the court, but they already referred to each other as "always soulmates," and she described him as "magic personified, love personified." His name, she said, is "etched into the depth of my soul."

Kelly Petty, Alaina's mother, described the late 14-year-old as a "very loving person."

"She loved her friends, she loved her family and, most importantly, she loved God," Kelly Petty said of her daughter. "I am heartbroken that I won't be able to watch her become the amazing young woman she was turning into."

Alain's sister Meghan echoed that sentiment, telling the court, "I would have loved to see her grow up. She would have been a blessing to the world."