Under the CGST Act, 2017, every registered taxpayer must file GSTR-3B by the 20th of the following month (or quarterly under the QRMP scheme). GSTR-1 for outward supplies is due by the 11th of the following month; the Invoice Furnishing Facility (IFF) for QRMP filers is due by the 13th.

Late filing of GSTR-3B attracts a penalty of Rs 50 per day (Rs 25 CGST + Rs 25 SGST), capped at Rs 10,000 per return. For nil returns, the penalty is Rs 20 per day. Interest at 18 percent per annum applies on outstanding tax under Section 50.

Input Tax Credit under Section 16 of the CGST Act can only be claimed if the supplier has filed their GSTR-1, the invoice appears in the recipient's GSTR-2B, and the recipient has received the goods or services. Rule 36(4) restricts Input Tax Credit claims to invoices appearing in GSTR-2B.

E-invoicing is mandatory for businesses with aggregate turnover exceeding Rs 5 crore from August 2023. E-invoices must be generated on the Invoice Registration Portal (IRP) within 30 days of the invoice date.

GSTR-9 annual return is mandatory for all regular taxpayers above the Rs 2 crore aggregate turnover threshold. GSTR-9C self-certified reconciliation statement is required for taxpayers with turnover above Rs 5 crore.

GST registration is mandatory when aggregate turnover exceeds Rs 40 lakh for goods (Rs 20 lakh for services, Rs 10 lakh for special category states) under Section 22 of the CGST Act.