1. GSTR-1 — what you sold
This is the return of outward supplies. B2B invoices are reported individually with the buyer's GSTIN; B2C sales are reported in summary, with larger inter-state B2C invoices treated separately. Credit and debit notes go in, as does an HSN-wise summary of what you sold. Because your buyers' input credit depends on this filing, a late or incorrect GSTR-1 is not just your problem — it is theirs, and they will call.
2. GSTR-3B — the summary and the payment
This is where outward tax and input credit meet and the balance is paid. It is a summary rather than an invoice-level return, which makes it quick to file and easy to file wrongly, because a rounded or estimated figure passes the form without objection. The tax is actually paid here, so an error in 3B is an error in what you paid.
3. Due dates and the quarterly option
Monthly filers have one set of dates; smaller taxpayers can opt for quarterly returns with monthly payment under the QRMP scheme, which changes the rhythm considerably. Which suits you depends on your turnover and your buyers — some business customers prefer suppliers who file monthly, because it affects when their credit appears. Confirm the current dates and your eligibility with your CA, as both have changed.
4. Late fees and what they escalate into
Late fees accrue per day per return, with a cap, and interest runs on tax paid late. Beyond the money, sustained non-filing blocks your e-way bill generation, which stops goods moving — for a material supplier that is a business-stopping consequence rather than a financial one. It also blocks your buyers' credit, which costs you customers.
5. Three checks before you file
Compare total sales in your summary against your own sales report for the month; a gap means bills were raised outside the system. Check that input credit is roughly what you expected; a shortfall almost always means purchase bills sitting unentered. And check that no B2B invoice is missing a buyer GSTIN, because those land in the wrong bucket or get rejected. Five minutes, and it catches the errors a notice would otherwise find.
6. Why the returns are easy when the records are right
None of this is difficult if every bill and every purchase was recorded as it happened, with the rate and HSN code on the item and the GSTIN on the party. Then the return data is a report. The three days most shops lose every month is not spent filing — it is spent reconstructing figures that already existed somewhere.