A Mastic Beach man testified Monday that his adoptive father sexually abused him for nearly a decade, starting the day he moved into Cesar Gonzales-Mugaburu’s home at the age of 13.

The man, now 23, said Gonzales-Mugaburu abused him almost every day in the Ridge house — sometimes before school and sometimes after.

“I became his favorite because he liked touching me and stuff,” the man told jurors in Suffolk County Court in Riverhead, where Gonzales-Mugaburu is on trial for predatory sexual assault and other charges.

Recounting the alleged abuse with little emotion, the man — whom Newsday is not identifying — testified that the first time Gonzales-Mugaburu sexually abused him he was in the sixth grade.

“How did it make you feel?” Suffolk Assistant District Attorney Laurie Moroff asked.

“Powerless,” he said. “I couldn’t do anything.”

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Gonzales-Mugaburu, 60, is charged with sexually abusing his foster and adopted sons and a dog. The most serious charge carries a maximum sentence of 25 years to life.

He was arrested January 2016 and has pleaded not guilty before County Court Judge Barbara Kahn.

Defense attorney Donald Mates Jr. of Hauppauge said in his opening statement that his client did not sexually abuse anyone. He said Gonzales-Mugaburu had been a foster parent to more than 100 boys over two decades and had never been arrested on charges of molesting his children before.

Monday, the former foster child testified he didn’t dare tell anyone — not his teachers, nor his therapists or social worker — of the sexual abuse because he said he was afraid of Gonzales-Mugaburu, and didn’t think anyone would believe him.

“I felt like if I told anyone, they wouldn’t believe my story,” he said. “They’d look at me like I’m crazy.”

The man said one of his foster brothers saw Gonzales-Mugaburu molesting him on two separate occasions, but he testified that he urged that brother not to say anything. That brother corroborated the man’s story last week when he testified that he twice walked in on Gonzales-Mugaburu having sex with the man.

Once, when he was 17, the man said he couldn’t take the sexual abuse anymore and ran away from home and lived in a homeless shelter for about a week before returning. The day before he turned 21, he said Gonzales-Mugaburu summoned him for sex, and pulled him off his bunk bed when he refused.

“I couldn’t deal with it no more and I left,” said the man, who said he went to live with friends.

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The man has filed a lawsuit against SCO Family of Services, the Glen Cove nonprofit agency that placed many children with Gonzales-Mugaburu on behalf of New York City and other agencies. He admitted under questioning by Mates that some of the information in the suit was not true, such as his claim that Gonzales-Mugaburu tried to give him Viagra or other medications.

He also acknowledged under questioning that he had twice been arrested and charged in thefts from department stores.