Ms Darinne Ko, the star witness at the centre of a sex-for-grades trial, arrived at the Subordinate Courts at 8.45am on Monday.

She will return to the witness stand to give evidence for a third day as the prosecution seeks to make its case against law professor Tey Tsun Hang.

Deputy Public Prosecutor Andre Jumabhoy on Friday applied to impeach her, as her testimony in court was "a material departure" from statements she had given to the Corrupt Practices Investigation Bureau (CPIB) last April.

He pointed out eight instances of such discrepancies to Chief District Judge Tan Siong Thye, with three addressed last Friday.

On the first day of the trial, which started Thursday, Ms Ko had painted a picture of love which flourished for a time between her and Tey, who was once her professor.

Tey, who arrived in court at 9.05am, is facing six charges of corruptly obtaining gifts and sex from her in exchange for better grades. The alleged offences took place between May and July 2010.

The gifts included a $740 Montblanc pen, two tailored shirts worth $236.20, a $160 iPod touch, and a $1,278.60 payment for a dinner Tey hosted. The sex took place in his NUS office on July 24 and 28, 2010.

The trial starts at 9.30am.

Tey, who is conducting his defence, may have the chance to cross-examine Ms Ko later today.