Learn Finnish: Vastasit, että huono sää pilasi festivaalin. - You answered that the bad weather ruined the festival.

Vastata (to answer) is a Type 4 verb. The present tense stem is vastaa-. To make it past tense (imperfect), you replace the final -a- of the stem with the past tense marker -si-, giving you vastasi-. Finally, add the personal ending -t for you to make vastasit.

Why is the past tense of pilata just pilasi? Is there a missing ending?

Pilata (to ruin) is also a Type 4 verb. Just like before, we take the stem pilaa-, drop the last vowel, and add -si- to get the past stem pilasi-. Because the subject is huono sää (a third-person it), we don't add any extra personal ending. The stem pilasi is the complete word!

Why does festivaali have an -n at the end?

Festivaalin is the object of the sentence because it is the thing being ruined. In Finnish, when an action is fully completed to a whole object, we use the total object form. For singular nouns, this form looks exactly like the genitive case, which means we add an -n to the end.

I remember Type 4 verbs often have consonant changes. Why doesn't that happen here?