The Finnish tax return is a pre-completed form with information received directly from third parties including employers, other payers of income, and many other sources of information.
The envelope that arrives in your registered postal address contains the form with the Finnish/Swedish heading: "ESITÄYTETTY VEROILMOITUS / FÖRHANDSIFYLLD SKATTEDEKLARATION".
Use this empty English-language form for taking notes. Additionally, you can fill it out if you received no Pre-completed tax return, and you have to submit your tax return independently. You can also fill it out to report your child's income and deductions. If you have nothing to add or correct regarding your last-calendar-year taxes, you do not have to send anything back.