A determinate sentence of 10 years means the prisoner is automatically paroled after 5 and remains on licence (i.e can be recalled to prison) for 5 years.
An indeterminate sentence of 3 years means he may never come out, or could be paroled after 3 years, 5 years, 10 years, 20 years (depending on his behaviour inside), and could remain on licence for the rest of his life.