Learn Finnish: Opiskelija menee isoon tehtaaseen. - The student goes into the big factory.

They are in the illative case, which means "into". In Finnish, adjectives must match the case of the noun they describe. Because the student is going into the factory (tehtaaseen), the word for big (iso) must also be in the "into" form (isoon).

Does the verb menee always require this "into" form?

Yes! The verb mennä (to go) requires the destination to be in a destination case like the illative. While in English you might just say you are going "to" the factory, in Finnish you say you are going "into" (isoon tehtaaseen) the building.

How is the illative ("into") form made for the word iso?

For most words ending in a basic short vowel, you make the illative form by simply doubling that last vowel and adding -n. Therefore, iso (big) becomes isoon.

Why does tehdas change so much to become tehtaaseen?

The word tehdas (factory) belongs to a specific group of words ending in -as. To add endings to these words, the -as changes to -aa-, creating the stem tehtaa-. Because this stem ends in a long vowel (two of the same vowels), the illative ending becomes -seen, giving us tehtaaseen.