Learn Finnish: Minä tiedän, että tuo roisto on tyhmempi. - I know that that villain is stupider.

The base word is tyhmä (stupid). In Finnish, when a two-syllable adjective ends in -a or -ä, that final vowel changes to an -e- before you add the comparative suffix -mpi. This is why tyhmä becomes tyhmempi.

Why are there two different words for 'that' (että and tuo) right next to each other?

English uses 'that' for two different grammatical roles, but Finnish keeps them separate. Että is a conjunction used to connect clauses ('I know that...'). Tuo is a demonstrative word used to point out a specific noun ('...that villain').

Is the comma before että mandatory?

Yes! Finnish punctuation rules are much stricter than English ones when it comes to commas. You must always use a comma to separate a main clause from a subordinate clause. Because että starts a subordinate clause, it almost always has a comma before it.

Could I drop the word Minä and just say Tiedän?

Yes, absolutely. Because the verb tiedän ends in -n, it already tells the listener that 'I' am the one doing the action. Including Minä just adds a bit of emphasis, like saying 'I know, even if others do not.'