Learn Finnish: Ovikello soi, ja hän avasi jännittävän paketin. - The doorbell rang, and he opened an exciting package.

It means "rang" (past tense), but it looks exactly the same as the present tense! For the verb soida (to ring or sound), the third-person singular is soi in both the present and the past tense. We know it is past tense here because the other verb in the sentence, avasi (opened), is in the past tense.

Why does avata (to open) become avasi?

Avata is a verb type 4 (verbs ending in -ta or -tä). To form the past tense, the -t- in the infinitive often changes to an -s- before adding the past tense -i. So, instead of using the present tense stem avaa-, we use avasi-.

Why do we use the -n ending for jännittävän paketin?

This is the total object (accusative case that looks like the genitive). Because he successfully opened the entire package and the action is complete, the object takes the -n ending. If he was in the middle of opening it, we would use the partitive case instead (jännittävää pakettia).

How would I add the word for "some" (jokin) to the phrase jännittävän paketin?

You would use jonkin. The pronoun jokin is usually used for things rather than people. Because the object in this sentence requires the -n ending, the pronoun must match the case of the noun. The phrase would become jonkin jännittävän paketin (some exciting package).