Learn Icelandic: Maðurinn er oft leiðinlegur.

Icelandic doesn't have a separate word for "the" like English does. Instead, it attaches a suffix to the end of the noun. Here, the base word maður (man) gets the masculine definite suffix -inn, becoming maðurinn (the man).

Why does the adjective leiðinlegur end in "-ur"?

Adjectives in Icelandic have to match the gender, number, and case of the noun they describe. Because maðurinn is a masculine, singular noun in the nominative (subject) case, the adjective takes the standard masculine nominative ending -ur.

How would I use this sentence with the Mér finnst... (I find...) phrase from this lesson?

You could say Mér finnst maðurinn oft leiðinlegur (I think the man is often boring / I find the man often boring). Even though mér is in the dative case (the "quirky subject" from this lesson), maðurinn is the subject of the thing you are describing, so it and its adjective leiðinlegur stay in the nominative case.

Can leiðinlegur mean anything other than "boring"?

Yes! While it often translates to "boring," it is frequently used in spoken Icelandic to mean "mean," "annoying," or "unpleasant" when talking about a person's behavior.