While still on probation for attacking women by covering their heads with plastic bags, an 18-year-old student tried to rape a woman at a void deck in his neighbourhood after watching pornography online.

Marcus Yow Kai Wen was yesterday sentenced to the mandatory minimum of eight years' jail and 12 strokes of the cane after he pleaded guilty in the High Court to a charge of attempted rape.

Three other sexual offence charges were taken into consideration.

In sentencing, Justice Choo Han Teck told Yow: "There is obviously something wrong with you because you have been committing this kind of offence since you were 16.

"If you do not do something about it and seek help... the next time you appear in court, you can be in lock-up for almost the rest of your adult life."

After he was sentenced, Yow's mother, who was in tears, collapsed and was helped by family members.

Yow was 16 when he assaulted four women, aged 18 to 34, by approaching them from behind and throwing a plastic bag or sweater over their heads.

He was put on probation of 18 months in December 2014 for using criminal force and ordered to seek psychiatric treatment.

Seven months into his probation, at about 3.20am on July 26 last year, he felt the urge to commit rape after watching pornographic videos with rape scenes at his home in Choa Chu Kang.

Wearing a cap and sunglasses and armed with a knife-shaped letter opener, he went downstairs to look for a victim.

He spotted a 25-year-old woman, who was on her way home, at a lift lobby of a nearby block. She noticed him and decided not to take the lift as she found it strange that he was wearing sunglasses.

But Yow pointed the letter opener at her and dragged her to the void deck of the next block of flats, where he tried to rape her, but did not succeed.

He then proceeded to perform degrading sexual acts on the victim who did as she was told out of fear that he would hurt her.

She broke free after she saw some passers-by and shouted for help.

Yow fled the scene, but left behind the letter opener and a slipper, which the victim retrieved.

She was helped to her block by the passers-by. She then called her boyfriend, who reported the case to the police.

Yesterday, Deputy Public Prosecutor Sellakumaran told the court that the attack resulted in the victim losing her self-esteem. She moved out of her parents' flat after the incident and the family's ties were strained.

Seeking a jail term on the "higher end", the DPP said the offences showed an escalation from Yow's prior brush with the law and that he was assessed to have a high risk of re-offending.

"It is evident that the accused had been so obsessed with rape pornography, that acts of rape were normalised to him," said the DPP, citing a psychological report that Yow was susceptible to the distorted beliefs of rape pornography.

Yow's lawyer, Mr Amarick Gill, said that Yow, a student at Institute of Technical Education College West, stopped his studies after flyers were circulated at his school and in the neighbourhood, publicising his earlier offences.

He had faced between eight and 10 years' jail and at least 12 strokes of the cane.