Learn Finnish: Sinun tulee allekirjoittaa veroilmoitus tänään.

The genitive + tulee + 1st infinitive construction is a more formal alternative to pitää and täytyy. You will frequently see it in official documents, contracts, and written instructions, which fits perfectly when talking about a tax return (veroilmoitus).

Why is the object veroilmoitus in its basic form instead of having an -n ending?

This is a crucial rule for necessity sentences. When a sentence uses a necessity verb (like tulee, pitää, or täytyy) and has a genitive subject (sinun), a total object does not take the normal -n accusative ending. It looks exactly like the basic nominative form.

Could I drop the pronoun sinun here, since Finnish often drops 'you' pronouns?

No, you must keep sinun in this sentence to specify 'you'. You can usually drop sinä when the main verb has the -t ending (like allekirjoitat), but necessity verbs like tulee stay frozen in the 3rd-person singular form. Without sinun, the sentence becomes a generic statement: 'One must sign the tax return.'

Is allekirjoittaa literally 'to write under'?