NUS law professor Tey Tsun Hang has started his cross-examination of former student Darinne Ko in the sex-for-grades case.

Addressing her in a gentle, fatherly manner, Tey began his line of defence by asking her about the circumstances surrounding her first statement to the Corrupt Practices Investigation Bureau (CPIB) made on April 2 last year.

Earlier, the prosecution completed its application to impeach Ms Ko on disparities between certain information given during her court testimony and that in her CPIB statements.

Tey faces six charges of corruptly obtaining gifts and sex from her in exchange for better grades. These allegedly 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 National University of Singapore (NUS) office on July 24 and 28, 2010.