Learn Finnish: Onko paitasi lyhyempi kuin minun paitani? - Is your shirt shorter than my shirt?

The basic word for 'short' is lyhyt. To add the comparative ending -mpi, we first need the word's stem. Words ending in -ut or -yt change that ending to -ue- or -ye-. So, lyhyt becomes the stem lyhye-, and then we add -mpi to get lyhyempi.

Why does the sentence say paitasi (your shirt) but then minun paitani (my shirt)? Can I just say sinun paitasi?

Yes, you can say sinun paitasi. In Finnish, the pronouns for 'my' (minun) and 'your' (sinun) can often be dropped because the possessive ending on the noun (-ni or -si) already tells us whose it is. The speaker here just chose to drop sinun but keep minun, which is completely natural, perhaps to add a slight emphasis to 'my shirt'.

What word is used for 'than' when comparing two things?

The word is kuin. You will use it very often with the -mpi comparative ending. The structure is just like English: lyhyempi kuin means 'shorter than'.

Why does the sentence start with Onko instead of On?