Determination of unpaid tax leads to notice requiring payment with interest and penalty unless taxpayer pays promptly to avoid penalty. Determination of unpaid, short paid, erroneously refunded tax or wrongly availed input tax credit (excluding fraud or wilful suppression) requires the proper officer to issue a show cause notice for tax, interest under section 50 and penalty; notices must be issued at least three months before the assessment time limit, a voluntary pre-notice payment bars further notice for that amount, payment within thirty days of notice avoids penalty, and the officer must issue a determination within three years from the annual return due date or from the date of erroneous refund.