Learn Finnish: Minä jään tänään sairaalaan. - I am staying in the hospital today.

In English, you stay in a place. But in Finnish, the verb jäädä (to stay or remain) acts like a movement verb. You are leaving yourself into a place! Because of this, it always requires the location to be in the illative (the 'into') case, which is why we use sairaalaan instead of sairaalassa.

How do you form the word sairaalaan?

The base word is sairaala (hospital). To form the illative ('into') case for most words ending in a short vowel, you double the last vowel and add -n. So, sairaala + a + n gives us sairaalaan.

Can I leave out the word Minä?

Yes, you can absolutely just say Jään tänään sairaalaan. The -n ending on the verb jään already makes it clear that 'I' am the one staying, so the pronoun minä is optional.

What is the basic dictionary form of the verb jään?

The dictionary form is jäädä (to stay). To make it 'I stay', you drop the -dä to get the stem jää-, and then add the personal ending -n.