Questions & Answers about Stilkur trésins er þykkari en sá jurtarinnar.
Why do we use sá here, and why is it in the masculine form?
Why is sá in the nominative case rather than the accusative?
When making comparisons with en (than), the case of the second item matches its role in the implied second half of the sentence. Since we are comparing the subject (stilkur) to another subject ("than that of the herb [is]"), sá must also be in the nominative case.
Why do we need the definite articles on the genitive words trésins and jurtarinnar?
Icelandic uses the definite article here to specify "of the tree" and "of the herb". If you dropped the articles and used the bare genitive forms trés and jurtar, it would change the meaning to "the stem of a tree is thicker than that of an herb".