Learn Icelandic: Þessi hegðun er mjög alvarleg.

While you often hear þetta as a general word for "this", here it directly modifies the noun hegðun (behavior). Since hegðun is a feminine noun, the demonstrative pronoun for "this" must match its gender, so we use the feminine form þessi.

Is there a trick to remembering the gender of hegðun?

Yes! Nouns that end in -un are almost always feminine in Icelandic. They are usually abstract concepts formed from verbs, just like how hegðun (behavior) comes from the verb að hegða (to behave).

Why doesn't the adjective alvarleg end in -ur like it does in the dictionary?

Adjectives must match the noun they describe in gender, number, and case. The dictionary always shows the masculine form (alvarlegur), but because hegðun is a feminine singular noun, we drop the -ur ending to get the feminine form alvarleg.

Does the word mjög (very) need to match the feminine noun too?

No, mjög is an adverb, not an adjective. While adjectives like alvarleg change their endings to match the noun's gender and number, adverbs never change their shape.