Can a president be forced to testify?

While the Supreme Court has never definitively ruled on the subject, the answer appears to be yes.

The question was tested during the Watergate scandal in 1974, when justices held unanimously that a president could be compelled to comply with a subpoena for tapes and documents. After the ruling, President Richard Nixon turned the materials over to prosecutors and then resigned.

Twenty-three years later, in allowing Paula Jones’ sexual harassment suit to go forward against President Bill Clinton, Justice John Paul Stevens wrote: “We have made clear that in a criminal case the powerful interest in the ‘fair administration of criminal justice’ requires that the evidence be given under appropriate circumstances lest the ‘very integrity of the judicial system’ be eroded.”
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In 1998, independent prosecutor Ken Starr served a subpoena on Clinton that ordered him to testify about his relationship with White House intern Monica Lewinsky. Clinton soon agreed to testify voluntarily, an arrangement Starr went along with because it headed off a potential challenge to the subpoena on constitutional grounds. - AP