"That of" and "Those of"
When comparing things in English, we often use phrases like "that of" or "those of" to avoid repeating a noun—for example, "The stem of the tree is thicker than that of the herb."
In Icelandic, you achieve exactly the same thing by using the demonstrative pronoun sá (that) followed by a noun in the genitive case. The pronoun you choose must agree in gender and number with the noun it is replacing.
Masculine: sá and þeir
If the noun you are replacing is masculine and singular, use sá. If the masculine noun is plural, use þeir (those).
| The stem of the tree is thicker than that of the herb. | Stilkur trésins er þykkari en sá jurtarinnar. |
| The thorns of the rose are harder than those of the herb. | Þyrnar rósarinnar eru harðari en þeir jurtarinnar. |
Neuter: það and þau
For neuter singular nouns, the equivalent pronoun is það. When substituting for a plural neuter noun, use þau.
| The leaf of this flower is larger than that of the herb. | Laufblað þessa blóms er stærra en það jurtarinnar. |
| The petals of this flower are red, but those of the herb are yellow. | Krónublöð þessa blóms eru rauð, en þau jurtarinnar eru gul. |
Feminine: sú and þær
Finally, if the noun being replaced is feminine and singular, use sú. (If you were replacing a feminine plural noun, the pronoun would be þær).
| The root of the plant is edible, but that of the tree is poisonous. | Rót plöntunnar er æt, en sú trésins er eitruð. |