Learn Finnish: Nuo sanat ovat ruudussa. - Those words are on the screen.

This is consonant gradation! When you add an ending like the inessive -ssa, it often weakens the consonants in the word. Here, the single strong t in ruutu weakens into a d. So, the stem becomes ruudu- before we add -ssa.

The translation says 'on the screen', but doesn't the -ssa ending mean 'in'?

Yes, the inessive case ending -ssa literally translates to 'in' or 'inside'. However, Finnish and English think about this location differently. In Finnish, text and pictures are considered to be 'in' the screen (ruudussa) or 'in' the TV (televisiossa) rather than 'on' it.

I learned that tuo means 'that'. Why is the sentence using nuo?

nuo is the plural form of tuo. Because the subject of the sentence is the plural sanat (words), you must use the plural word for 'those' (nuo) instead of the singular 'that' (tuo).

How do we know that sanat is plural?