Learn Icelandic: Þyrnar rósarinnar eru harðari en þeir jurtarinnar. - The thorns of the rose are harder than those of the herb.

Þeir is the masculine plural form of the demonstrative pronoun sá (that/the one). Here, it replaces the masculine plural noun þyrnar (thorns) to avoid repeating it. Combined with the genitive noun jurtarinnar, the phrase þeir jurtarinnar translates cleanly to "those of the herb".

Why do both rósarinnar and jurtarinnar end with the exact same suffix?

Both rós (rose) and jurt (herb/plant) are strong feminine nouns. To show possession ("of the rose" or "of the herb"), they must be in the genitive case. They both take the genitive singular ending -ar, followed by the feminine genitive definite article -innar.

Does the comparative adjective harðari change to match the masculine plural noun þyrnar?

No, comparative adjectives in Icelandic are much simpler than their base forms. Once an adjective is in the comparative form (usually ending in -ari or -ri), it uses a weak declension. In the plural, it will always end in -i regardless of the noun's gender or case.

The singular word for thorn is þyrnir. Why is the plural þyrnar instead of just keeping the "i"?