Learn Finnish: Illallinen on valmis kello seitsemän. - Supper is ready at seven o'clock.

When you use the word kello (clock) to state a time, the number stays in its basic nominative form, like seitsemän. If you leave out the word kello, you need the adessive case ending -ltä to express "at", making it seitsemältä. Both are perfectly natural and common ways to say "at seven" in Finnish.

Why is the adjective valmis in the basic nominative form rather than the partitive valmista?

In Finnish, the form of the describing adjective depends on the subject. Here, illallinen (supper) is treated as a complete, countable event, so the adjective takes the basic nominative form valmis. If the subject were an uncountable mass noun like ruoka (food) or keitto (soup), you would use the partitive case: Ruoka on valmista.

How is illallinen different from the other common word for dinner, päivällinen?

It usually comes down to the time of day. Illallinen is derived from the word ilta (evening) and refers to a later supper, usually eaten after 7:00 PM. Päivällinen is derived from päivä (day) and is the main warm meal typically eaten earlier, between 4:00 PM and 6:00 PM.