Learn Croatian: Dosadno mu je.

In Croatian, physical or mental states (like being bored, cold, or warm) are often expressed using the dative case. Instead of saying "He is bored", the Croatian sentence literally translates to "It is boring to him". This is why you use the dative clitic mu (to him).

Both mu and je are clitics (short words). Why is mu placed before je?

Croatian has a strict ordering rule for clitics. When you have a dative pronoun clitic (like mu) and a verb clitic (like je), the pronoun clitic always comes first.

Why does dosadno end in -o if we are talking about a man?

Because this sentence literally means "It is boring to him", there is no actual subject like "he" or "she" for the word to describe. In these "it is..." situations, Croatian uses the neuter singular ending, which is -o.

How would I change this sentence to say "I am bored" or "She is bored"?

Since the structure is "It is boring to [person]", you just need to swap mu for a different dative clitic! To say "I am bored", use mi (Dosadno mi je). To say "She is bored", use joj (Dosadno joj je).